fix: harden exporter shape and data handling

[JulienBalianSonos]

Summary

• handle HF configs where head_dim exists but is None
• allow split_with_sizes sections of length zero while still rejecting negative sections
• make TensorVariable equality safe for tensor subclasses, meta tensors, and numpy arrays
• add shape-inference rules for aten::_grouped_mm and aten::gather

Quality review

• This is intentionally separate from the MoE/HQQ export PR. These are generic exporter/IR robustness fixes and would make the MoE PR harder to review if folded in.
• The head_dim change overlaps behavior already present in PR [RDY] feat: add basic MoE support #55, but keeping it here is reasonable because it is independent and small. If [RDY] feat: add basic MoE support #55 lands first, this will either disappear in rebase or be a no-op conflict.
• The split change is narrow: only zero-sized sections are newly accepted; negative sizes still fail explicitly.
• The equality helper now avoids the previous ambiguous/unsafe (data == data).all() path for tensor subclasses and arrays. Base tensors still compare actual values; tensor subclasses compare metadata to avoid forcing subclass-specific equality semantics.
• The grouped-mm/gather infer rules avoid executing kernels during shape inference, which is safer for optional/private ATen ops and avoids overload-resolution surprises.

Tests

• uv run ruff format --check packages/llm/torch_to_nnef_llm/config.py packages/llm/tests/test_config.py torch_to_nnef/op/aten/split.py torch_to_nnef/torch_graph/ir_data.py torch_to_nnef/torch_graph/ir_op.py tests/test_grouped_mm_infer.py tests/test_ir_data_equality.py tests/test_aten_parser_fixes.py
• uv run ruff check packages/llm/torch_to_nnef_llm/config.py packages/llm/tests/test_config.py torch_to_nnef/op/aten/split.py torch_to_nnef/torch_graph/ir_data.py torch_to_nnef/torch_graph/ir_op.py tests/test_grouped_mm_infer.py tests/test_ir_data_equality.py tests/test_aten_parser_fixes.py
• uv run pytest packages/llm/tests/test_config.py -q
• uv run pytest tests/test_grouped_mm_infer.py tests/test_ir_data_equality.py tests/test_aten_parser_fixes.py -q