[Relax][PyTorch] Add decomposed operator support for Pad

[guan404ming]

Related Issue

• [Relax][PyTorch] Enable decomposition for unary ops and refactor tests #18401

Why

• When run_ep_decomposition=True is enabled, PyTorch decomposes pad operators into lower-level operations:
  • Constant mode → constant_pad_nd.default
  • Reflect/Replicate modes → index.Tensor with None indices
  • Circular mode → copy.default and slice operations
• Some of the decomposed operators were not supported, causing failures

How

• Added support for constant_pad_nd.default and copy.default operator
• Fixed _index_tensor to handle None indices by:
  • Using take operation when only one dimension is indexed (optimization)
  • Converting None to explicit arange for general cases
• Updated test_pad to use run_ep_decomposition=True