Fixed training for TF XLM
This PR fixes `model.fit()` training for TF XLM model, and tested in a script similar to `run_tf_glue.py`. It also is tested and works with AMP and tf.distribute for mixed precision and multi-GPU training.

This changes some Python `assert` statements to `tf.debugging.assert_equal` both in `TFXLMMainLayer.call()` and `gen_mask()`

Otherwise, errors encountered:
* `TypeError: You are attempting to use Python control flow in a layer that was not declared to be dynamic. Pass 'dynamic=True' to the class constructor.`
* `OperatorNotAllowedInGraphError: using a 'tf.Tensor' as a Python 'bool' is not allowed in Graph execution. Use Eager execution or decorate this function with @tf.function.`
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/1668?src=pr&el=h1) Report
> Merging [#1668](https://codecov.io/gh/huggingface/transformers/pull/1668?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/079bfb32fba4f2b39d344ca7af88d79a3ff27c7c?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `75%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/1668/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/1668?src=pr&el=tree)

```diff
@@          Coverage Diff           @@
##           master   #1668   +/-   ##
======================================
  Coverage    85.9%   85.9%           
======================================
  Files          91      91           
  Lines       13653   13653           
======================================
  Hits        11728   11728           
  Misses       1925    1925
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/1668?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [transformers/modeling\_tf\_xlm.py](https://codecov.io/gh/huggingface/transformers/pull/1668/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL21vZGVsaW5nX3RmX3hsbS5weQ==) | `90.39% <75%> (ø)` | :arrow_up: |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/1668?src=pr&el=footer). Last update [079bfb3...842f3bf](https://codecov.io/gh/huggingface/transformers/pull/1668?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>Awesome, thanks a lot @tlkh 