Adds INT2 (2-bit signed) and UINT2 (2-bit unsigned) data type

[vraspar]

Adds INT2 (2-bit signed) and UINT2 (2-bit unsigned) data types following ONNX spec PR #7446. These complement existing INT4/UINT4 support with similar packed representation.

Changes

Enums (_enums.py)

• Added INT2=25, UINT2=26 enum values
• Added ml_dtypes.int2/uint2 mappings and type properties (bitwidth=2, signed/unsigned)
• Added short names "i2"/"u2"

Packing (_type_casting.py)

• Implemented pack_2bitx4() / unpack_2bitx4() for 4-values-per-byte packing
• Handles non-multiple-of-4 sizes with padding

Core (_core.py)

• Updated PackedTensor to support 2-bit types (was 4-bit only)
• Added INT2/UINT2 to non-numpy-native types list
• Updated byte representation to pack 2-bit types

Serialization (serde.py)

• Added 2-bit unpacking from raw_data
• Added INT2/UINT2 to int32_data serialization paths

Torch adapters (tensor_adapters.py)

• Added torch.int2/uint2 dtype mappings for future compatibility
• PyTorch doesn't yet support creating tensors with these types

Example

import numpy as np
import ml_dtypes
import onnx_ir as ir

# Create and serialize INT2 tensor
array = np.array([-2, -1, 0, 1], dtype=ml_dtypes.int2)
tensor = ir.Tensor(array)
proto = ir.serde.to_proto(tensor)

# 4 elements packed into 1 byte
assert len(proto.raw_data) == 1

# Round-trip works
restored = ir.serde.from_proto(proto)
assert np.array_equal(restored.numpy(), array)

Original prompt

Helpe me create PR for this issue: #250

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[codecov]

Codecov Report

❌ Patch coverage is 73.21429% with 15 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.30%. Comparing base (17754ff) to head (83fc971).
⚠️ Report is 2 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/onnx_ir/tensor_adapters.py	40.00%	1 Missing and 5 partials ⚠️
src/onnx_ir/_core.py	69.23%	2 Missing and 2 partials ⚠️
src/onnx_ir/_enums.py	33.33%	4 Missing ⚠️
src/onnx_ir/serde.py	75.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #251      +/-   ##
==========================================
- Coverage   77.33%   77.30%   -0.04%     
==========================================
  Files          40       40              
  Lines        5042     5097      +55     
  Branches     1009     1027      +18     
==========================================
+ Hits         3899     3940      +41     
- Misses        853      860       +7     
- Partials      290      297       +7     

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[justinchuby]

@vraspar could you check the test errors? Thanks