[dmlc/tvm] [DOCS][COMMUNITY] add missing header for PMC in community doc (#3708)

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[TVM Discuss] [Development] How to register FTVMSchedule for new relay op

This is usually defined in Python. So for example, if you define your op’s call to `_make` in `python/tvm/relay/op/tensor.py`, the schedule and op pattern are usually defined in `python/tvm/relay/op/_tensor.py` There are lots of examples in the `_*.py` files. --- [Visit Topic](https://di