Fix T5 beam search when using parallelize
# What does this PR do?

As requested by @patrickvonplaten in conversation on issue https://github.com/huggingface/transformers/issues/9200, this fixes a crash when trying to use beam search on T5 models split across multiple GPUs using `model.parallelize()`. It uses the fix from https://github.com/huggingface/transformers/pull/9219, applied to the T5-specific code (also related is https://github.com/huggingface/transformers/pull/9596 which refactored the `_reorder_cache` functions).

I tested the fix on a t5-small model. Before:

```
from transformers import AutoTokenizer, AutoModelForSeq2SeqLM  
tokenizer = AutoTokenizer.from_pretrained("allenai/unifiedqa-t5-small")
model = AutoModelForSeq2SeqLM.from_pretrained("allenai/unifiedqa-t5-small")
device_map = {0: range(0,3), 1: range(3, 6)}
input_string = "What was the color of the sky?\\nIt was a dark stormy night."
input_ids = tokenizer.encode(input_string,return_tensors="pt").to("cuda:0")
output = model.generate(input_ids, num_beams=2)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/oyvindt/miniconda3/envs/transformers4/lib/python3.9/site-packages/torch/autograd/grad_mode.py", line 27, in decorate_context
    return func(*args, **kwargs)
  File "/home/oyvindt/miniconda3/envs/transformers4/lib/python3.9/site-packages/transformers/generation_utils.py", line 1044, in generate
    return self.beam_search(
  File "/home/oyvindt/miniconda3/envs/transformers4/lib/python3.9/site-packages/transformers/generation_utils.py", line 1788, in beam_search
    model_kwargs["past"] = self._reorder_cache(model_kwargs["past"], beam_idx)
  File "/home/oyvindt/miniconda3/envs/transformers4/lib/python3.9/site-packages/transformers/models/t5/modeling_t5.py", line 1635, in _reorder_cache
    layer_past_state.index_select(0, beam_idx),
RuntimeError: Input, output and indices must be on the current device
```
After:
```
...
output = model.generate(input_ids, num_beams=2)
tokenizer.batch_decode(output, skip_special_tokens=True)
 --> ['dark stormy']
```

As far as I know this small fix shouldn't have any adverse effects. As to why the tests added in  https://github.com/huggingface/transformers/pull/9219 didn't catch this, possibly that's because they're not generally run in multi-GPU setups? 

@OyvindTafjord  Hi, I am trying to figure out how to use model parallelization on T5 but having some problems. I tried to reproduce your result but got the following error:
```
from transformers import AutoTokenizer, AutoModelForSeq2SeqLM  
tokenizer = AutoTokenizer.from_pretrained("allenai/unifiedqa-t5-small")
model = AutoModelForSeq2SeqLM.from_pretrained("allenai/unifiedqa-t5-small")
device_map = {0: range(0,3), 1: range(3, 6)}
model.parallelize(device_map)
input_string = "What was the color of the sky?\\nIt was a dark stormy night."
input_ids = tokenizer.encode(input_string,return_tensors="pt").to("cuda:0")

output = model.generate(input_ids, num_beams=2)
```
```
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/guest/anaconda3/envs/huggingface_latest/lib/python3.6/site-packages/torch/autograd/grad_mode.py", line 15, in decorate_context
    return func(*args, **kwargs)
  File "/home/guest/anaconda3/envs/huggingface_latest/lib/python3.6/site-packages/transformers/generation_utils.py", line 922, in generate
    model_kwargs = self._prepare_encoder_decoder_kwargs_for_generation(input_ids, model_kwargs)
  File "/home/guest/anaconda3/envs/huggingface_latest/lib/python3.6/site-packages/transformers/generation_utils.py", line 417, in _prepare_encoder_decoder_kwargs_for_generation
    model_kwargs["encoder_outputs"]: ModelOutput = encoder(input_ids, return_dict=True, **encoder_kwargs)
  File "/home/guest/anaconda3/envs/huggingface_latest/lib/python3.6/site-packages/torch/nn/modules/module.py", line 550, in __call__
    result = self.forward(*input, **kwargs)
  File "/home/guest/anaconda3/envs/huggingface_latest/lib/python3.6/site-packages/transformers/models/t5/modeling_t5.py", line 897, in forward
    inputs_embeds = self.embed_tokens(input_ids)
  File "/home/guest/anaconda3/envs/huggingface_latest/lib/python3.6/site-packages/torch/nn/modules/module.py", line 550, in __call__
    result = self.forward(*input, **kwargs)
  File "/home/guest/anaconda3/envs/huggingface_latest/lib/python3.6/site-packages/torch/nn/modules/sparse.py", line 114, in forward
    self.norm_type, self.scale_grad_by_freq, self.sparse)
  File "/home/guest/anaconda3/envs/huggingface_latest/lib/python3.6/site-packages/torch/nn/functional.py", line 1724, in embedding
    return torch.embedding(weight, input, padding_idx, scale_grad_by_freq, sparse)
RuntimeError: arguments are located on different GPUs at /opt/conda/conda-bld/pytorch_1587428091666/work/aten/src/THC/generic/THCTensorIndex.cu:403
```
* My current environment:
transformers: 4.7.0.dev0
torch: 1.5.0

Could you please help me to figure out the problem and give me some direction that I should start with? I don't have much experience with model parallelization, do I need to modify the `input_ids`?

Thanks in advance.
<|||||>@bing0037 Hm, I tested with 4.7.0 now and the above code works for me. I noticed my initial set of commands was missing the critical `model.parallelize(device_map)` step, but looks like you made sure to include that? 

You could double check that `model.encoder.first_device` returns the expected `'cuda:0'`, and then the code at https://github.com/huggingface/transformers/blob/master/src/transformers/models/t5/modeling_t5.py#L870 should make sure the embeddings are also on that same device, so you shouldn't get that line 897 error above.<|||||>@OyvindTafjord Thank you for your reply. The problem was the inconsistency of my command and the above command works well.
BTW, the above command is for parallelized model **inference**, could you please give me some suggestions for parallelized model **training**? 
Currently, I am trying to finetune **t5-large** model using `run_summarization.py` on multiple GPUs by using model parallelization. 
* My test 1: By adding ```model.parallieze()``` directly in `run_summarization.py`, but got the following error:
```
    model = AutoModelForSeq2SeqLM.from_pretrained(
        model_args.model_name_or_path,
        from_tf=bool(".ckpt" in model_args.model_name_or_path),
        config=config,
        cache_dir=model_args.cache_dir,
        revision=model_args.model_revision,
        use_auth_token=True if model_args.use_auth_token else None,
    )
    

+    device_map = {0: [0, 1, 2],
+                1: [3, 4, 5, 6, 7, 8, 9],
+                3: [10, 11, 12, 13, 14, 15, 16],
+                4: [17, 18, 19, 20, 21, 22, 23]}
+    model.parallelize(device_map) # Splits the model across several devices

    model.resize_token_embeddings(len(tokenizer))

    if model.config.decoder_start_token_id is None:
        raise ValueError("Make sure that `config.decoder_start_token_id` is correctly defined")
```
```
Traceback (most recent call last):
  File "run_summarization.py", line 616, in <module>
    main()
  File "run_summarization.py", line 540, in main
    train_result = trainer.train(resume_from_checkpoint=checkpoint)
  File "/home/guest/anaconda3/envs/huggingface_latest/lib/python3.6/site-packages/transformers/trainer.py", line 1300, in train
    args.max_grad_norm,
  File "/home/guest/anaconda3/envs/huggingface_latest/lib/python3.6/site-packages/torch/nn/utils/clip_grad.py", line 30, in clip_grad_norm_
    total_norm = torch.norm(torch.stack([torch.norm(p.grad.detach(), norm_type) for p in parameters]), norm_type)
RuntimeError: All input tensors must be on the same device. Received cuda:0 and cuda:7
```

* My test 2: I referred to this question: https://discuss.huggingface.co/t/transformers-4-6-0-on-sagemaker/6217/4, but still can't change the model parallesim status:
```
pip install git+https://github.com/aws/sagemaker-python-sdk.git
pip install sagemaker
```
```
>>> from transformers.file_utils import is_sagemaker_mp_enabled
>>> is_sagemaker_mp_enabled()
False
```
Could you give me some resources that I could refer to? Thank you!<|||||>@bing0037 I haven't tried the parallelize functionality in the context of training, so I'm not of much help on that. 