Add aten_hardtanh_backward function (#1715)

Depends on #1707, will add unit test after #1707 merged.

Author: xiaowuhu

onnxscript/function_libs/torch_lib/ops/nn.py

@@ -632,12 +632,15 @@ def aten_hardtanh(self: TReal, min_val: float = -1.0, max_val: float = 1.0) -> T
     return op.Clip(self, min_val, max_val)
 
 
+@torch_op("aten::hardtanh_backward", trace_only=True)
 def aten_hardtanh_backward(
     grad_output: TensorType, self: TensorType, min_val: float, max_val: float
 ) -> TensorType:
     """hardtanh_backward(Tensor grad_output, Tensor self, Scalar min_val, Scalar max_val) -> Tensor"""
 
-    raise NotImplementedError()
+    max_mask = op.Where(op.Greater(self, max_val), 0.0, 1.0)
+    min_mask = op.Where(op.Less(self, min_val), 0.0, 1.0)
+    return op.Mul(op.Mul(grad_output, max_mask), min_mask)
 
 
 def aten_huber_loss(