[EncoderDecoder Tests] Improve tests

[patrickvonplaten]

Currently when executing:
pytest tests/test_modeling_encoder_decoder.py all BERT tests are run as well due to problems with the import of BertModelTester (see PR #3383).

This PR takes the PR: #4027 and does some minimal changes in the encoder decoder test file.

I think @sshleifer found a great solution to circumvent that by moving the BertModelTester class out of the BertModelTest class.

What do you think @julien-c , @LysandreJik ?

[codecov-io]

Codecov Report

Merging #4046 into master will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4046      +/-   ##
==========================================
+ Coverage   78.84%   78.85%   +0.01%     
==========================================
  Files         114      114              
  Lines       18689    18689              
==========================================
+ Hits        14735    14737       +2     
+ Misses       3954     3952       -2     

Impacted Files	Coverage Δ
src/transformers/modeling_tf_utils.py	92.93% <0.00%> (+0.16%)	⬆️
src/transformers/file_utils.py	73.55% <0.00%> (+0.41%)	⬆️

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[LysandreJik]

This looks great! What do you think @julien-c ?

[julien-c]

Small nit but otherwise looks good to me

[julien-c]

By the way, do you guys want to do this for all other ModelTester objects @patrickvonplaten and @sshleifer? And maybe (if relevant) define a common abstract class they inherit from?

[patrickvonplaten]

By the way, do you guys want to do this for all other ModelTester objects @patrickvonplaten and @sshleifer? And maybe (if relevant) define a common abstract class they inherit from?

I think this would be a good idea. We can probably clean up the tests a lot this way! I will note it down on my To-Do-List

[sshleifer]

LMK if you want to split it up @patrickvonplaten

[patrickvonplaten]

LMK if you want to split it up @patrickvonplaten

I tihnk I won't find time in the next 2 weeks to do this - feel free to start working on it if you want :-)