Fix all offload and MP tests
# What does this PR do?

This should fix all GPU tests for model parallelism and offload. I stopped trying to make the model bigger since it was causing so many failures (unrelated to current failing tests). Then there was a tiny bug in `from_pretrained` when tied wieghts are present, as highlighted by the failure of the GPT-Neo offload tests. Adding the tie weights before computing the auto device map is necessary to know which weights are tied when auto-generated this device map.

The OPT model for testing was too tiny for the MP/offload tests (that's why there was some logic to make tiny models bigger there) so I just adjusted its size (speed is not really affected).

Finally the T5 tests starting failing after the tied weights because the decoder has to be tied with the shared layer, which require bigger models than the tiny one used for test. Here I didn't make the model bigger since the tests already take some time, so I adjusted the percentages used for the total model size in those tests. I added a new class variable for that, but happy to overwrite the tests in the T5 modeling test if you prefer it.
_The documentation is not available anymore as the PR was closed or merged._