Rewrite concatenate([x, x]) as repeat(x, 2)

[tchan102]

Add optimization for Join → Repeat when concatenating identical tensors

Description

This PR introduces a graph rewrite optimization in pytensor/tensor/rewriting/basic.py that replaces redundant Join operations with an equivalent and more efficient Repeat operation when all concatenated tensors are identical.

Example:
join(0, x, x, x) → repeat(x, 3, axis=0)

Key additions:

• Implemented new rewrite function local_join_to_repeat registered under both @register_canonicalize and @register_specialize.
• Added corresponding test test_local_join_to_repeat to verify correctness, performance, and behavior for vectors and matrices.

Related Issue

• [ x] Closes Rewrite concatenate([x, x]) as repeat(x, 2) #1710

Checklist

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

Type of change

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

[ricardoV94]

Let's try with @register_canonicalize only

[ricardoV94]

Btw would be nice to get rid of this join (and split) symbolic axis if you would like to work on that after this PR. relevant issue: #1528