Add support for PyTorch 2.6

[akihironitta]

Adds support for PyTorch 2.6.

Fixes type issues raised by mypy with PyTorch 2.6:

Found 6 errors in 4 files (checked 87 source files)
torch_frame/nn/encoding/cyclic_encoding.py:31: error: "Tensor" not callable  [operator]
torch_frame/nn/conv/excelformer_conv.py:95: error: Value of type "Union[Tensor, Module]" is not indexable  [index]
torch_frame/nn/conv/excelformer_conv.py:96: error: Value of type "Union[Tensor, Module]" is not indexable  [index]
torch_frame/nn/models/tabnet.py:261: error: "Tensor" not callable  [operator]
torch_frame/nn/models/tabnet.py:263: error: "Tensor" not callable  [operator]