Add unstack op

[OriolAbril]

Description

First pass at unstack. It is working already, need to sort out tests and double check the order in which unstack happens.

Related Issue

• Related to Sketch of dim-ed tensors pymc-devs/pytensor#407

Checklist

• Checked that the pre-commit linting/style checks pass
• Included tests that prove the fix is effective or that the new feature works
• Added necessary documentation (docstrings and/or example notebooks)
• If you are a pro: each commit corresponds to a relevant logical change

Type of change

• New feature / enhancement
• Bug fix
• Documentation
• Maintenance
• Other (please specify):

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📚 Documentation preview 📚: https://pytensor--1.org.readthedocs.build/en/1/

[OriolAbril]

@ricardoV94 let me know if the PR should have been done in a different way and how the code looks. As I commented in the test code itself, tests currently pass but I am only checking matching shapes with xarray, the actual elements are different. I have to figure out if the idea I had of testing the complementary operation to circumvent the fact that xarray's unstack needs coordinates can't actually be used or if I am inverting some stack(new_dim=["a", "b"] while the other has ["b", "a"].

[ricardoV94]

Are the unstack dims in the same order as xarray?

[ricardoV94]

Closing in favor of pymc-devs#1412