Have FiniteMPS.{left, right}_envs use BaseMPS.apply_transfer_operator

[AidanGG]

I might be missing something, but I believe the contractions in FiniteMPS.left_envs are identical to those in BaseMPS.left_transfer_operator. I suggest the alteration to make use of the latter's JITting.

I can open a PR if this would offer some improvement.

[mganahl]

Thanks @AidanGG, great suggestion! A PR would be very much appreciated!