[Fix][Relax][Frontend][Torch] Compare shapes without == on symbolic dims in the reshape shortcut

[hiyufan]

Problem

_reshape skips an identity reshape with

if current_shape is not None and list(current_shape) == list(dims):
    return x

On a symbolic dimension == builds a PrimExpr instead of answering, and Python then asks it for a truth value:

ValueError: Cannot use and / or / not operator to Expr, hint: use tvm.tirx.all / tvm.tirx.any, ...
  File "python/tvm/relax/frontend/torch/base_fx_graph_translator.py", line 2559, in _reshape
  File "python/tvm/ir/expr.py", line 347, in __bool__

It only surfaces when the ranks match, because list equality compares lengths first — which is why x.reshape(x.shape[0], -1) on a rank-3 input imports and x.reshape(x.shape[0], 0, x.shape[0]) raises:

expression	input	torch	frontend on main
x.reshape(x.shape[0], 0, x.shape[0])	(batch, 0, 4)	(batch, 0, batch)	ValueError
x.reshape(0, x.shape[1], x.shape[1])	(0, batch, 4)	(0, batch, batch)	ValueError
x.reshape(x.shape[0], -1)	(batch, 0, 4)	(batch, 0)	ok (rank differs)

This is the "40 pre-existing ValueError cases" I reported as out of scope in #20255 — I had assumed they were below the frontend. They are this line.

Fix

_same_dims compares dimension by dimension: static dims as integers, symbolic dims with tvm_ffi.structural_equal, a static-vs-symbolic pair as different. A genuine identity with a symbolic batch is still recognised (x.reshape(x.shape[0], 2, 4) on (batch, 2, 4) emits no reshape, asserted in the test). An expression written differently on the two sides (s*2 vs 2*s) is treated as different, which costs a no-op reshape and never a wrong shape.

Verification

The reshape sweep from #20255 (938 reshape/view/flatten/unflatten targets over inputs mixing symbolic and zero dims, output shape compared with torch.export's, symbol names canonicalised):

	matched	mismatched	of which raising
main	892	46	40
this PR	932	6	0

Per-case diff: no case fails with this change that did not fail before it; 40 repaired. The 6 left are the sweep's own renderer printing s*2 and 2*s differently (verified in #20255's thread), not the frontend.

test_frontend_from_exported_program.py and test_frontend_from_fx.py: failure sets identical before and after apart from the new test. ruff check / ruff format --check (v0.12.3) clean.

Test

test_reshape_symbolic_target_same_rank — the first row above builds and matches torch under a dynamic batch, and the identity case emits no reshape. Fails against the previous head with the ValueError above.

Independent of #20372–#20376 (branched from main). Touches the same _reshape as #20255 (merged), one line below it.

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This change was prepared with AI assistance (Claude). I have reviewed and verified it, and can speak to it in review.

[cchung100m]

LGTM, thanks to @hiyufan 😄