Add nn.Module as superclass
Add nn.Module as superclass of `MMBTModel` to fix #4532 
Are there upstream issues?  I can't see how my one-liner is breaking these tests<|||||># [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4533?src=pr&el=h1) Report
> Merging [#4533](https://codecov.io/gh/huggingface/transformers/pull/4533?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/e19b978151419fe0756ba852b145fccfc96dbeb4&el=desc) will **increase** coverage by `0.00%`.
> The diff coverage is `100.00%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/4533/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/4533?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #4533   +/-   ##
=======================================
  Coverage   77.86%   77.87%           
=======================================
  Files         123      123           
  Lines       20566    20566           
=======================================
+ Hits        16014    16015    +1     
+ Misses       4552     4551    -1     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/4533?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_mmbt.py](https://codecov.io/gh/huggingface/transformers/pull/4533/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19tbWJ0LnB5) | `22.11% <100.00%> (ø)` | |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4533/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `88.66% <0.00%> (ø)` | |
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4533/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `73.85% <0.00%> (+0.41%)` | :arrow_up: |

------

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> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4533?src=pr&el=footer). Last update [e19b978...42d544b](https://codecov.io/gh/huggingface/transformers/pull/4533?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>Re-ran the tests and they pass, this was a transient (connectivity?) error.

This PR looks reasonable to me. I'll just cc @suvrat96 for information