DPL (`tvm.relax.dpl`) is heavily used across the TVM stack — operator
fusion, CUTLASS/cuBLAS/cuDNN backend dispatch, and user-defined graph
transforms all rely on it. Since there is no doc explaining how to use
it, this pr adds deep-dive documentation
(`docs/deep_dive/relax/dpl.rst`) covering DPL's pattern construction,
matching, rewriting APIs, and integration with `FuseOpsByPattern`
backend dispatch passes.
Shushi Hong