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（空气呼啸）
(air whooshing)

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- 你代码中的一些错误非常简单。
- Some bugs in your code are very straightforward.

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你尝试运行它，某处出现语法错误，
You try running it, you get a syntax error somewhere,

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Python 准确地告诉你在哪里，然后你修复它。
Python tells you exactly where, and you fix it.

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这很棒，简单而且令人满意。
This is great, it's simple and it's satisfying.

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但有时，事情会崩溃
Sometimes, though, things crash

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错误是无法理解的。
and the error is impossible to understand.

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由于一些原因，这种情况在机器学习中经常发生，
This happens a lot in machine learning for a few reasons,

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你正在处理大数据结构，
you're working with big data structures,

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你正在使用这些大而复杂的库
you're using these big, complex libraries

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有很多活动部件，
with a lot of moving parts,

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而且你正在做大量的 GPU 计算，
and also you're doing a lot of GPU computing,

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而且一般来说调试起来要困难得多。
and that in general is much more difficult to debug.

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在 Keras 中还有一个问题
In Keras there's the additional problem

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你的模型通常在执行前编译，
that your models are often compiled before execution,

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这对性能很有帮助
which is great for performance

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但这也使调试它们变得非常困难。
but it makes debugging them very difficult as well.

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所以，这将是一个关于该怎么做的视频
So, this is going to be a video about what to do

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当你遇到那些噩梦般的一个错误时
when you run into one of those nightmare bugs

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而且你只是不知道从哪里开始修复它。
and you just have no idea where to begin with fixing it.

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所以，给你一些直觉
So, to give you some intuitions for

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最常见的错误
the most common things that go wrong

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并导致这些奇怪的问题，
and cause these weird issues,

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并告诉你在哪里寻找错误的来源
and show you where to look for the sources of bugs

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你遇到的，让我们使用这个示例脚本。
that you encounter, let's use this example script.

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因此，我将在这里分两部分向你展示。
So, I'll show it to you here in two parts.

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首先，我们做所有的导入，我们加载一个数据集，
First, we do all our imports, we load a dataset,

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我们创建了 tokenizer 并对数据集进行分词。
we create our tokenizer and we tokenize the dataset.

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接下来，我们将数据集转换为 TensorFlow 数据集，
Next, we convert our datasets to TensorFlow datasets,

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这就是 tf.data.Dataset，
so that's tf.data.Dataset,

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这样我们就可以拟合它们，
and that's so that we can run fit on them,

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然后我们从预训练的 checkpoint 加载我们的模型，
and then we load our model from a pretrained checkpoint,

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我们对其进行编译，并将其与这些数据集相匹配。
we compile it and we fit it with those datasets.

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所以，这看起来很简单，
So, this seems straightforward enough,

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这与我们之前在课程中所做的类似。
it's similar to what we've done in the course before.

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但请注意，这是令人毛骨悚然的代码
But beware, this is spooky code

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并隐藏着许多黑暗而神秘的秘密。
and hides many dark and mysterious secrets.

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那么，当我们运行它时会发生什么？
So, what happens when we run it?

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好吧，这不是很好。
Well, it's not great.

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所以，我们收到了这个错误信息，但它是什么意思呢？
So, we get this error message, but what does it mean?

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我们试图训练我们的数据，但我们没有梯度？
We tried to train on our data, but we got no gradient?

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这很令人困惑，我的意思是，我们从何开始
It's pretty perplexing, I mean, how do we even begin

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调试没有得到梯度？
to debug not getting a gradient?

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所以，当你得到的错误没有立即提示
So, when the error you get doesn't immediately suggest

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问题出在哪里，最好的解决办法
where the problem is, the best solution

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通常是按顺序遍历事物，
is often to walk through things in sequence,

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确保在每个阶段输出看起来都是正确的，
making sure at each stage that the outputs look right,

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那时一切看起来都很好。
that everything looks okay at that point.

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而且，当然，这意味着开始的地方
And, of course, that means the place to start

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总是检查你的数据。
is always to check your data.

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所以，最好的办法是确保
So, the best way to make sure

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你给模型的数据是好的，
that the data you're giving the model is good,

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是从 tf.data.Dataset 中抓取一批
is to grab a batch from the tf.data.Dataset

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你的模型正在训练，
that your model is training on,

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那是因为它就在最后
and that's because it's right at the end

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的数据 pipeline 。
of the data pipeline.

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所以这意味着如果这些输出是好的，
And so that means that if those outputs are good,

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你可以保证你的数据 pipeline 运行良好。
you're guaranteed that your data pipeline is working well.

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所以，我们可以通过遍历数据集来做到这一点
So, we can do that by looping over the dataset

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进行一次迭代然后中断，
for one iteration and then breaking,

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这给了我们一个分批。
and that gives us a single batch.

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那么，当我们检查该批次时，我们得到了什么？
So, what do we get when we inspect that batch?

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我们会看到我们没有得到任何梯度
We'll see that we're not getting any gradient

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因为我们没有将标签传递给 Keras。
because we're not passing labels to Keras.

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所以，我们的标签在批次中，
So, our labels are in the batch,

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但它们是输入字典中的键
but they're a key in the input dictionary

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而且它们不像 Keras 期望的那样是一个单独的标签，
and they're not a separate label as Keras expects,

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所以这是你会遇到的最常见的问题之一
so this is one of the most common issues you'll encounter

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使用 TensorFlow 训练 Transformers 模型时。
when training Transformers models with TensorFlow.

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我们的模型都可以在内部计算损失，
Our models can all compute loss internally,

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但要将损失用于训练
but to use that loss for training

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标签需要在输入字典中传递，
the labels need to be passed in the input dictionary,

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其模型可以看到它们。
where the model can see them.

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这种内部损失是我们使用的损失
This internal loss is the loss that we use

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当我们在调用编译时没有指定损失时，
when we don't specify a loss when we call compile,

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当我们不指定损失参数时。
when we don't specify a loss argument.

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所以，另一方面，Keras，
So, Keras, on the other hand,

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通常期望标签单独传递
usually expects labels to be passed separately

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从输入字典，
from the input dictionary,

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并且对模型不可见，
and not to be visible to the model,

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损失计算通常会失败
and loss computations will usually fail

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如果你不那样做
if you don't do that

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所以我们需要选择其中之一，
So we need to choose one or the other,

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要么我们使用模型的内部损失
either we use the model's internal loss

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并将标签保留在原处，
and keep the labels where they are,

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或者我们继续使用 Keras 损失
or we keep using Keras losses

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但我们将标签移动到 Keras 期望的地方。
but we move the labels to the place Keras expects them.

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所以，还是简单点，让我们解决这个问题
So, or simplicity here, let's fix this issue

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通过使用模型的内部损失，
by using the model's internal losses,

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我们通过删除损失参数来做到这一点
and we do that by removing the loss argument

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从调用编译。
from the call to compile.

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那么，如果我们现在尝试训练会发生什么？
So, what happens if we try training now?

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所以我们用它重新编译，我们调用 model.fit，会发生什么？
So we recompile with that, we call model.fit, what happens?

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好吧，这次它运行了，但现在我们消失了 NaN。
Well, it runs this time but now we get a loss of NaN.

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所以，这不好，NaN 表示不是数字
So, that's not good, NaN means not a number

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总的来说，这不是一个好损失。
and it's not a good loss to have in general.

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事实上，如果我们现在检查我们的模型，
In fact, if we inspect our model now,

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我们会看到不仅所有的输出都是 NaN，
we'll see that not only are all the outputs NaN,

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所有的权重和损失也是 NaN。
all the weights are NaN as well, as well as the loss.

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所以一旦一个 NaN 悄悄爬进你的计算，
So once a single NaN creeps into your computations,

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它倾向于传播，因为它从损失传播
it tends to spread, because it propagates from the loss

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一旦它处于损失状态，它就会处于梯度状态，
and once it's at the loss it's at the gradient,

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它达到梯度，
it gets to the gradient,

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然后一旦它处于梯度中
and then once it's in the gradient

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它进入权重更新，
it enters the weight updates,

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然后你所有的权重更新最终也都是 NaN 。
and then all your weight updates end up as NaN as well.

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所以 NaN 在这里完全破坏了我们的模型，
So NaN just completely destroyed our model here,

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但它首先潜入何处？
but where did it creep in first?

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所以要找出答案，我们需要回到一个点
So to find out, we need to back to a point

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在模型被摧毁之前，
before the model was destroyed,

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我们需要重新初始化模型
we need to re-initialize the model

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并查看第一批的输出。
and look at the outputs for just the first batch.

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当我们这样做时，
And when we do that,

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我们看到 NaN 首先出现在损失中，
we see that NaN first appears in the loss,

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但仅在某些样本中。
but only in some samples.

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所以你可以更详细地看到这个
So you can see this in more detail

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在课程笔记的随附部分中，
in the accompanying section of the course notes,

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我在这里移动得相当快，
I am moving fairly quickly here,

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但我们发现如果我们看一下标签，
but we find that if we look at the labels,

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损失为 NaN 的样本的标签均为 2。
the samples with a loss of NaN all have a label of two.

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所以这给了我们一个非常有力的线索，
So this gives us a very strong clue,

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如果我们使用 model.config.num_labels 检查模型，
if we check the model with model.config.num_labels,

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我们看到模型认为只有两个标签，
we see that the model thinks there's only two labels,

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但如果我们看到值为二，
but if we see a value of two,

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这意味着至少有三个标签
that means there's at least three labels

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因为 0 也是一个标签。
because 0 is a label as well.

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所以我们得到大量 NaN
So we got a lots of NaN

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因为我们的标签集中有一个 “不可能” 的标签，
because we got an "impossible" label in our label set,

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并修复我们需要返回并设置模型
and to fix that we need to go back and set the model

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期待正确数量的标签，
to expect the right number of labels,

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所以我们可以设置 num_labels=3
so we can set num_labels=3

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当我们初始化模型用 from_pretrained 时，
when we initialize the model but from_pretrained,

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现在希望我们可以避免这个问题。
and now hopefully we can avoid this issue.

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所以，现在我们认为我们的数据很好，我们的模型也很好
So, now we think our data is good and our model is good

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所以训练应该有效
and so training should work

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但是如果我们尝试运行 model.fit，我们，嗯……
but if we try running model.fit, we, well...

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我的意思是，我们确实有损失，
I mean, we do get a loss,

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这是一个数字，它正在下降
it is a number and it is going down

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但它不会很快下降
but it's not going down very quickly

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如果我们一直运行这个，
and if we keep running this out,

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我们会发现它停在相当高的损失值处。
we'll find that it stalls at a fairly high loss value.

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发生什么了？
So, what's going on?

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好吧，当一切正常时，
Well, when things are mostly working,

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但训练只是很慢或有点奇怪，
but training is just slow or a bit odd,

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这通常是查看优化器的好时机
that can often be a good time to look at your optimizer

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和你的训练超参数。
and your training hyperparameters.

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这就是我想提的地方
And this is where I want to mention

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最常见的问题来源之一
one of the most common sources of issues

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当你使用 Keras 时，
when you're working with Keras,

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你可以用字符串命名优化器，
you can name things like optimizers with strings,

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所以 Keras 支持它，而且非常方便，
so Keras supports that and it's very convenient,

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但如果你这样做所有的选择
but if you do that all of the options

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默默地设置为默认值。
get silently set to their default values.

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所以我们将优化器指定为 Adam，
So we specified our optimizer as Adam,

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但在这个过程中我们无形中得到了
but in the process we invisibly got

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默认学习率，即 1e-3，
the default learning rate, which is 1e-3,

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或 10 的 -3 次方。
or 10 to the power of -3.

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所以这个学习率太高了
So this learning rate is way too high

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用于训练 transformer 模型，
for training transformer models,

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我们应该回去直接指定学习率，
we should go back and specify the learning rate directly,

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不使用字符串。
not using a string.

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所以，这里的好值在 1e-5 和 1e-4 之间
So, good values here are between 1e-5 and 1e-4

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所以让我们平分差价并选择 5e-5。
so let's split the difference and pick 5e-5.

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所以如果你用那个重新编译，
So if you recompile with that,

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你最终会发现训练确实有效。
you'll find that training actually works, at last.

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损失有效减少
The loss goes down efficiently

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并且收敛到一个较低的值。
and it converges to a lower value.

168
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所以，再一次，我确实很快地完成了这个
So, again, I did go through this quite quickly

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我强烈建议查看课程笔记
and I strongly recommend checking out the course notes

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要更详细地了解这一点，
to see this in more detail,

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并自己试验代码
and to experiment with the code yourself

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看看错误是什么样的
and see what the errors look like

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以及如何解决他们，
and how you can approach them,

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但我希望我在这里给了你一个简短的总结
but I hope I've given you here a quick summary

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最常见的错误
of the most common bugs

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也许是最常见的调试方法
and maybe the most common debugging approaches

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来对付他们。
to dealing with them.

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所以，祝你好运，记得多休息
So, good luck, and remember to take plenty of breaks

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如果你的代码给你带来困难。
if your code is giving you a hard time.

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（空气呼啸）
(air whooshing)

