Fix PyTorch 2.13 named overload compatibility

[juicyzhou]

Fixes #101.

Summary

• avoid dereferencing named-tensor ATen overloads that were removed in PyTorch 2.13
• remove named-tensor entries from the decomposition list
• keep the two autocast registrations only when an older PyTorch version still provides those overloads
• add regression coverage for importing torchax and constructing the default environment
• add a focused PyTorch 2.13 compatibility CI job while keeping the existing PyTorch 2.8 full test matrix unchanged

Root cause

PyTorch 2.13 removed named tensors and their ATen overloads. torchax.amp accessed two removed overloads at module import time, and torchax.decompositions referenced another 20 removed overloads while constructing the default environment. Accessing any missing overload raises AttributeError before torchax can be used.

The removed decomposition entries are now deleted directly. The two autocast entries are registered conditionally so older PyTorch versions retain their existing behavior.

Validation

• PyTorch 2.8.0, Python 3.12: pytest -q test/test_torch_compat.py test/test_amp.py — 6 passed
• PyTorch 2.13.0, Python 3.12: pytest -q test/test_torch_compat.py test/test_amp.py — 6 passed
• PyTorch 2.13.0: import torchax and torchax.default_env() succeed
• ruff check and ruff format --check pass for all changed Python files

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