Edit a way to get `projected_context_layer`
I edited a way to get `projected_context_layer`.

Instead of doing
'''
w = (
    self.dense.weight.t()
    .view(self.num_attention_heads, self.attention_head_size, self.hidden_size)
    .to(context_layer.dtype)
)
b = self.dense.bias.to(context_layer.dtype)
projected_context_layer = torch.einsum("bfnd,ndh->bfh", context_layer, w) + b
'''

I added `self.merge_last_ndims` at `AlbertAttention`.
'''
def merge_last_ndims(self, x, n_dims):
    s = x.size()
    assert n_dims > 1 and n_dims < len(s)
    return x.view(*s[:-n_dims], -1)
'''

I commited this one yesterday, but that one didn't pass the tests, so I re-wrote my code and re-committing now.
Hi, please check the [contribution guidelines](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md#start-contributing-pull-requests) for the code quality tests to pass.

Why did you edit this, does this solve a bug or add new functionality?<|||||>Hi i edited this because i read a comment like..
“ # Should find a better way to do this “
Not because of bug or new features<|||||># [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2602?src=pr&el=h1) Report
> Merging [#2602](https://codecov.io/gh/huggingface/transformers/pull/2602?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/23c6998bf46e43092fc59543ea7795074a720f08?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/2602/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/2602?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #2602      +/-   ##
==========================================
+ Coverage   74.61%   74.61%   +<.01%     
==========================================
  Files          87       87              
  Lines       14802    14804       +2     
==========================================
+ Hits        11044    11046       +2     
  Misses       3758     3758
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/2602?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_albert.py](https://codecov.io/gh/huggingface/transformers/pull/2602/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19hbGJlcnQucHk=) | `79.03% <100%> (+0.11%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/2602?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2602?src=pr&el=footer). Last update [23c6998...a96e39e](https://codecov.io/gh/huggingface/transformers/pull/2602?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
