fix: canonicalize singleton partition-view axes for GT lowering

[TaoTao-real]

Summary

• Add a safe canonicalization in memref.subview -> GlobalTensor lowering for partition-view outputs.
• Reorder only statically-proven singleton axes (size == 1) ahead of non-singleton axes when building GT shape/stride metadata.
• Keep dynamic axes as non-singleton (no speculative reorder).
• Add regression test covering both positive and negative paths.

Motivation

• Fixes [Bug] partion_tensor_view在高维tensor场景下会触发pto-isa assert #453.
• pto-isa ND2NZ/DN2NZ paths require GT to satisfy 2D-like constraints (leading dims must be 1 in 5D right-aligned form).
• Certain partition-view shapes such as [1, 16, 1, 16] were semantically legal but emitted as non-canonical 5D forms that can trip backend assertions.

Design

• Location: SubviewToEmitCPattern in lib/PTO/Transforms/PTOToEmitC.cpp.
• Safety boundary:
  • Reorder is enabled only when the number of non-singleton axes is <= 2.
  • Only axes statically known as singleton are moved.
  • Non-singleton axis relative order is preserved.
• This is metadata canonicalization only (shape/stride ordering); pointer/offset address computation is unchanged.

Testing

• Added: test/basic/issue453_partition_view_singleton_axis_reorder.pto
  • Positive: [1,16,1,16] lowers to Shape<1,1,1,16,16>.
  • Negative: [2,16,1,16] remains non-reordered (Shape<1,2,16,1,16>).
• Ran:
  • build/tools/ptoas/ptoas --pto-arch=a5 test/basic/issue453_partition_view_singleton_axis_reorder.pto | FileCheck ...
  • build/tools/ptoas/ptoas test/basic/infer_layout_ambiguous_minor2d_user_dn.pto | FileCheck ...
  • build/tools/ptoas/ptoas --pto-arch=a3 test/basic/tstore_forms_emitc.pto | FileCheck ...

Risk / Rollback

• Risk: low, scoped to subview->GlobalTensor metadata construction.
• Rollback: revert this PR to restore previous lowering behavior.

[gemini-code-assist]

Code Review

This pull request introduces a singleton axis reordering optimization for partition views in PTOToEmitC.cpp, allowing axes with a size of 1 to be moved to the front when safe. It also includes a new test case to verify this behavior. I have no further feedback to provide, as the suggested improvements for using standard library algorithms are optional and the current implementation is clear and correct.

[TaoTao-real]

Update with the refined scope discussed:

• singleton-axis canonicalization for partition-view subview is now selectively enabled only when:
  1. consumer is pto.tload
  2. source GlobalTensor layout is ND or DN
  3. destination tile config is NZ-like
• it no longer runs globally for all subviews.

Guard coverage added in test/basic/issue453_partition_view_singleton_axis_reorder.pto:

• positive: ND -> NZ-like tload still canonicalizes
• negative: >2 non-singleton dims still do not canonicalize
• guard Python wheel packaging for ptoas, in manylinux docker image #1: ND -> ND tload does not canonicalize
• guard (docker) fix platform on manylinux wheel #2: tstore-only user does not canonicalize

Local validation:

• ninja -C build ptoas
• build/tools/ptoas/ptoas --pto-arch=a5 test/basic/issue453_partition_view_singleton_axis_reorder.pto | FileCheck --check-prefix=A5
• build/tools/ptoas/ptoas test/basic/infer_layout_ambiguous_minor2d_user_dn.pto | FileCheck
• build/tools/ptoas/ptoas --pto-arch=a3 test/basic/tstore_forms_emitc.pto | FileCheck --check-prefix=A3

[TaoTao-real]

Follow-up update based on review direction (clean layering):

• Moved singleton-axis canonicalization logic out of PTOToEmitC.cpp into a dedicated pass:
  • pto-canonicalize-subview-for-tload
  • implementation file: lib/PTO/Transforms/PTOCanonicalizeSubviewForTLoad.cpp
• Pipeline placement:
  • after PTOViewToMemref
  • before EmitC lowering
• PTOToEmitC now only consumes a marker attr (pto.singleton_axis_permutation) and applies the precomputed permutation; strategy/gating logic is no longer embedded in codegen lowering.

Gating semantics remain the same:

• trigger only when all effective consumers are pto.tload
• source GlobalTensor layout is ND/DN
• destination tile config is NZ-like
• and only safe singleton-axis movement (<=2 non-singleton dims)

Additional test coverage added:

• DN -> NZ-like tload positive case in test/basic/issue453_partition_view_singleton_axis_reorder.pto

Validation rerun (all pass):

• ninja -C build ptoas
• ptoas --pto-arch=a5 test/basic/issue453_partition_view_singleton_axis_reorder.pto | FileCheck --check-prefix=A5
• ptoas test/basic/infer_layout_ambiguous_minor2d_user_dn.pto | FileCheck
• ptoas --pto-arch=a3 test/basic/tstore_forms_emitc.pto | FileCheck --check-prefix=A3