[Relax] Normalize negative concat axis in ReorderPermuteDimsAfterConcat

[cchung100m]

Hi Committers,

This PR fixes #19575.

Root Cause

ReorderPermuteDimsAfterConcat reads concat axis and uses it as an index into the permutation axes.

When concat(axis=-1) is used, the negative axis was converted directly to size_t before indexing, which can produce an out-of-range index and crash (e.g. IndexError: Index -1 out of bounds 4).

Solution

In src/relax/transform/reorder_permute_dims_after_concat.cc:

1. Read concat axis as signed integer first.
2. Normalize negative axis with axis += ndim.
3. Add explicit range checks after normalization.
4. Use the normalized axis for permutation-axis remapping

This keeps behavior unchanged for non-negative axes and only fixes negative-axis handling.

[gemini-code-assist]

Code Review

This pull request updates the reorder_permute_dims_after_concat.cc file to implement axis normalization and bounds checking for concatenation operations. A critical issue was identified where the new implementation was added alongside the old one, resulting in variable redefinitions and a compilation error.