Implement pack/unpack Ops

[jessegrabowski]

Description

This came up in the context of #1550 . There are many cases where we want to be able to ravel a sequence of inputs into a single vector, do some computation, then reshape the outputs back into the original shapes. As was pointed out in #1552, this is exactly what einops pack/unpack does.

It would be great for us to have similar functionality, using numeric axes instead of the letter-based API.

I also tagged this as xtensor, because it makes sense for this functionality to also have an xtensor implementation (allowing you to pack/unpack using named dims)

[ricardoV94]

xtensor already has this functionality, with stack and unstack

[ricardoV94]

https://pytensor.readthedocs.io/en/latest/library/xtensor/type.html#pytensor.xtensor.type.XTensorVariable.unstack

[jessegrabowski]

Took off the xtensor tag in that case