Enable pegasus fp16 by clamping large activations
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+ If encoder layer is about to return inf, clamp to a float **near** the boundary of fp16 range
+ This improves rouge for distil-pegasus-xsum-12-12 in fp16 by 0.1 ROUGE 2
+ I cannot think of how this change could make things worse.
+ Other models' metrics are unaffected!
Hi,

Any chance of implementing the same for the "pegasus-cnn_dailymail" (distilled) model?

Regards,
Karthik<|||||>@karthikgali this PR implements for all models that inherit from Bart. This includes pretty much every `sshleifer/` checkpoint and every `pegasus/` checkpoint.

Pegasus variants with 16 encoder layers still do not work well in fp16, but I will push a 12 layer distilled version soon that does!