Make BART more ONNX friendly
**Major concerns:** 
1. ONNX complains about Python raw integer usage.
2. ONNX doesn't support boolean indexing with something else than vector. The code was using 2D Tensor indices (batch, token). 

**PR workarounds:**
1. Remove the call to `len(..)` and prefer the use of `.size(-1)`
2. Attempt to index the output tensors to retrieve only the **last** EOS token over the sequence axis through `.index_select(..)`
I believe @mfuntowicz ran the slow tests on this one and there were no failures, but this PR is so cryptic I don't understand what's happening.<|||||>This issue has been automatically marked as stale and been closed because it has not had recent activity. Thank you for your contributions.

If you think this still needs to be addressed please comment on this thread.