nn.Linear with BSR inputs: spare the user from explicit Triton kernel registrations (#98403)
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### <samp>🤖 Generated by Copilot at 08f7a6a</samp>
This pull request adds support for triton kernels in `torch` and `torch/cuda`, and refactors and tests the existing triton kernel for BSR matrix multiplication. It also adds a test case to ensure that importing `torch` does not implicitly import `triton`.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/98403
Approved by: https://github.com/malfet, https://github.com/cpuhrsch
