CLI: convert sharded PT models
# What does this PR do?

This PR adds a major upgrade and a minor change to the `pt-to-tf` CLI.

Major upgrade: we can now convert sharded PT models. It updates how the `from_pt` loading works so as to be able to load from shards. It also updates how the `pt-to-tf` CLI stores the models, so it uses sharding capabilities when needed.

Minor change: adds a flag to control the maximum hidden layer admissible error. It is relatively common to find models where the outputs from the PT and TF models are nearly the same, but the hidden layers have a larger mismatch. This flag allows us to temporarily increase the admissible error if the model seems to be behaving properly (for instance, all RegNet models had a hidden layer difference between 1e-4 and 1e-2, but the outputs were behaving properly).

Example of sharded TF model PR, using the updated tools: https://huggingface.co/facebook/regnet-y-10b-seer-in1k/discussions/1 
_The documentation is not available anymore as the PR was closed or merged._<|||||>BTW could we add 2 tests, `test_load_sharded_tf_to_pt` and `load_sharded_pt_to_tf` <|||||>TF shards -> PT probably won't work, but I will add the test for PT shards -> TF 👍 