Add doc for adding custom ops (#2509)

Summary:
Add documentation for adding custom ops.

Pull Request resolved: #2509

Reviewed By: xuzhao9

Differential Revision: D64497281

Pulled By: FindHao

fbshipit-source-id: 20f4096ebbce53c7d9a713cacbde016c521aa7c3

Author: FindHao

torchbenchmark/operators/readme.md

@@ -0,0 +1,85 @@
+## How to add a custom operator benchmark
+
+1. Create a new folder in the `operators` directory.
+2. Add an `operator.py` and `__init__.py` file to the new folder.
+3. Implement the `Operator` class.
+4. Register the operator benchmarks in the `operator.py` file.
+
+### Example
+
+```
+operators/
+    my_operator/
+        __init__.py
+        operator.py
+```
+
+## `__init__.py`
+
+The `__init__.py` file only needs to import the operator to trigger the registration of the benchmarks.
+
+```
+from .operator import Operator
+```
+
+## `operator.py`
+
+The `operator.py` file needs to implement the following:
+
+1. `Operator` class: This class should inherit from `BenchmarkOperator`.
+2. `get_input_iter`: This method should return an iterator of input examples for the
+   operator.
+3. `@register_benchmark`: This decorator should be used to register the benchmarks for
+   the operator.
+4. `get_bwd_fn`: This method should return a callable that performs the backward pass
+   for the operator when needed.
+5. `get_grad_to_none`: This method should be overridden to set the gradients to your argument for
+   the operator when needed.
+
+### Example
+
+```
+from torchbenchmark.util.benchmark_registry import register_benchmark
+import triton
+class Model(torch.nn.Module):
+    def __init__(self):
+        super().__init__()
+        self.linear = torch.nn.Linear(10, 10)
+
+    def forward(self, x):
+        return self.linear(x)
+
+@triton.jit
+def _kernel(XXX):
+    # your triton kernel implementation
+    pass
+
+def kenrel_wrapper(a, b, activation=""):
+    M, K = a.shape
+    K, N = b.shape
+    # Allocates output.
+    c = torch.empty((M, N), device=a.device, dtype=a.dtype)
+    # 1D launch kernel where each block gets its own program.
+    grid = lambda META: (
+        triton.cdiv(M, META["BLOCK_SIZE_M"]) * triton.cdiv(N, META["BLOCK_SIZE_N"]),
+    )
+    _kernel[grid](XXX)
+    return c
+
+class Operator(BenchmarkOperator):
+    def __init__(self, tb_args: argparse.Namespace, extra_args: Optional[List[str]] = None):
+        super().__init__(tb_args, extra_args)
+        self.model = Model()
+
+    def get_input_iter(self) -> Generator:
+        for i in range(10):
+            yield torch.randn(10)
+
+    @register_benchmark(baseline=True)
+    def my_operator(self, input) -> Callable:
+        return lambda: self.model(input)
+
+    @register_benchmark()
+    def my_operator2(self, input) -> Callable:
+        return lambda: kernel_wrapper(input)
+```