Copilot/fix issue 250 handling

[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]

❌ 1 Tests Failed:

Tests completed	Failed	Passed	Skipped
941	1	940	0

View the top 1 failed test(s) by shortest run time

src.onnx_ir._core_test.PackedTensorTest::test_initialize_raises_when_dtype_not_4bit

Stack Traces | 0.003s run time

src\onnx_ir\_core_test.py:2939: in test_initialize_raises_when_dtype_not_4bit
    self.assertIn("PackedTensor only supports INT4, UINT4, FLOAT4E2M1", str(cm.exception))
E   AssertionError: 'PackedTensor only supports INT4, UINT4, FLOAT4E2M1' not found in 'PackedTensor only supports INT2, UINT2, INT4, UINT4, FLOAT4E2M1, but got FLOAT'

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[justinchuby]

I realized we don’t need to handle endianess because these are subtbyte types. So endianess is irrelevant