Make autocast a Transform

Currently autocast (transforms/autocast.py) is carried over from the old-style transforms on callable and invoked like

jit(autocast(fn))

it would be good to make this a AutocastTransform to be invoked as

jit(fn, transforms=[Autocast()])

for better composability.

This should be vaguely doable with the transforms e.g. as explained in Thunder Sessions Episode 7 plus the bsym mapping provided by the existing mechanism, so it might be a good first or second issue.