Avoid default allocation for taps of length 1 in ScanSaveMem

[ricardoV94]

The check we had for whether a variable was a default scan buffer always failed for single tapped outputs. There's a conservative check that the original value is not being broadcast to the number of initial taps, but that doesn't matter for single taps.

Also added some checks that we are actually only keeping buffers of the expected size in the test.

---

📚 Documentation preview 📚: https://pytensor--1395.org.readthedocs.build/en/1395/

[Copilot]

Pull Request Overview

This PR addresses an issue with the default scan buffer allocation for single-tapped outputs in ScanSaveMem and enhances the tests for buffer size validation. Key changes include:

• Adjusting the test configuration by excluding "scan_pushout" and renaming an internal function from f_rnn to step for clarity.
• Updating the implementation of default scan buffer handling by adding a new parameter (taps) to _is_default_scan_buffer and adapting buffer expansion and slicing logic accordingly.

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 4 comments.

File	Description
tests/scan/test_rewriting.py	Modified test configuration and assertions regarding scan buffer sizes and function naming.
pytensor/scan/rewriting.py	Updated _is_default_scan_buffer's signature and revised buffer handling logic using the taps value.

[ricardoV94]

this would fail with the infamous [-0:] edge case