fix derived scan logprob when observed provides more broadcastable information

[eclipse1605]

Description

fixed a failure in derived scan logprob construction when the observed/value tensor provides more static broadcastability information than the generative scan graph (e.g. observed has a size-1 axis like (date, 1) while the scan state was inferred as non broadcastable on that axis).

in this, model.logp() could fail during the measurable scan rewrite with a scan outputs_info broadcast pattern mismatch (scan output inferred as matrix like vs. outputs_info expecting vector-like).

• the scan logprob rewrite propagate/align broadcastability metadata introduced by observed/value variables so the rewritten scan is internally consistent.
• this happens without turning scan inner placeholders into non nominal Apply nodes (so scan reconstruction remains valid).

note

I think the same idea can be generalized by treating static broadcastability metadata as part of the measurable scan rewrite contract:

• identify the “outer” value/observed variables that participate in the measurable rewrite and extract their broadcastable axes (ignoring the time axis for sequences).
• when creating inner scan placeholders / outputs_info proxies for the logprob rewritten scan, ensure their TensorType.shape reflects any size-1/broadcastable axes implied by the outer variables.
• apply the same normalization to tapped init buffers (pt.join / outputs_info) so that init and scan outputs agree on broadcastability, without inserting broadcast Apply nodes into the inner graph (placeholders must remain nominal vars).

Related Issue

• Closes #7892

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):