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.

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📚 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.

[Copilot]

Ensure that all callers of _is_default_scan_buffer supply the correct 'taps' value so that the default buffer check correctly distinguishes between single and multiple taps.

Suggested change

	def _is_default_scan_buffer(x: TensorVariable, taps: int) -> bool:
	def _is_default_scan_buffer(x: TensorVariable, taps: int) -> bool:
	"""
	Determine if a scan buffer is the default buffer.
	Parameters:
	x (TensorVariable): The tensor variable to check.
	taps (int): The number of taps (time steps) associated with the buffer.
	Must be correctly supplied by the caller to ensure accurate checks.
	Returns:
	bool: True if the buffer is the default scan buffer, False otherwise.
	"""

[Copilot]

Verify that deriving 'taps' from init_l[i] accurately reflects the intended tap count, and that this value is consistently used to compute extra_size in buffer expansion.

Suggested change

	if idx < op_info.n_mit_sot + op_info.n_sit_sot:
	if idx < op_info.n_mit_sot + op_info.n_sit_sot:
	# Validate init_l[i] before using it to derive taps
	if not isinstance(init_l[i], int) or init_l[i] < 0:
	raise ValueError(f"Invalid tap count in init_l[{i}]: {init_l[i]}")

[Copilot]

Confirm that updating the slice boundary to use 'taps' (instead of init_l) maintains the intended behavior for buffer trimming in ScanSaveMem.

Suggested change

	taps = init_l[op_info.n_mit_mot + idx]
	taps = taps[op_info.n_mit_mot + idx]

[Copilot]

Confirm that excluding 'scan_pushout' aligns with the intended optimization behavior and does not conflict with other scan optimizations.