Fix convert to tf
I struggled with this same problem for a long time.  The naive `assign` op way, puts all of the weights into both the checkpoint file (`.ckpt.data-XXXXX-of-YYYYY`) and the meta file (`.ckpt.meta`).  This is because assign adds an operation to the graph.  So basically, you have two instructions in your meta file, one that initializes the variable with random values and then another that assigns the pytorch values to these tensors.  But really, you want to initialize everything once with the meta file and then read the data file which should have your pytorch weights in it.  Tensorflow hides this functionality deep within it's source code and every answer on stackoverflow tells one to use `assign`.  But the `tf.keras.backend.set_value` function does simply replace the weights of a variable.  However, this function makes some assumptions about your session and your graph so I had to change your code a bit.  Long story short, doing easy things in tensorflow is hard.

So what's the difference?

1. the meta file will be about 1mb instead of 400+ mb
2. the script runs in about 10 seconds instead of 3 minutes



 
# [Codecov](https://codecov.io/gh/huggingface/pytorch-transformers/pull/907?src=pr&el=h1) Report
> Merging [#907](https://codecov.io/gh/huggingface/pytorch-transformers/pull/907?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/pytorch-transformers/commit/46cc9dd2b51a152b2e262ec12e40dddd13235aba?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/pytorch-transformers/pull/907/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/pytorch-transformers/pull/907?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master     #907   +/-   ##
=======================================
  Coverage   79.03%   79.03%           
=======================================
  Files          34       34           
  Lines        6234     6234           
=======================================
  Hits         4927     4927           
  Misses       1307     1307
```



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<|||||>Ok thanks David!