Add api info for torch._C._nn.pyi (pytorch#162936)

Fix part of pytorch#148404

APis involved are as followed:

- silu
- silu_
- smooth_l1_loss
- soft_margin_loss
Pull Request resolved: pytorch#162936
Approved by: https://github.com/FFFrog, https://github.com/ezyang

Author: orangeH25

tools/pyi/gen_pyi.py

@@ -772,6 +772,47 @@ def gen_nn_functional(fm: FileManager) -> None:
                     "Tensor",
                 )
             ],
+            "silu": [
+                defs(
+                    "silu",
+                    [
+                        INPUT,
+                    ],
+                    "Tensor",
+                )
+            ],
+            "silu_": [
+                defs(
+                    "silu_",
+                    [
+                        INPUT,
+                    ],
+                    "Tensor",
+                )
+            ],
+            "smooth_l1_loss": [
+                defs(
+                    "smooth_l1_loss",
+                    [
+                        INPUT,
+                        "target: Tensor",
+                        "reduction: str = ...",
+                        "beta: float = 1.0",
+                    ],
+                    "Tensor",
+                )
+            ],
+            "soft_margin_loss": [
+                defs(
+                    "soft_margin_loss",
+                    [
+                        INPUT,
+                        "target: Tensor",
+                        "reduction: str = ...",
+                    ],
+                    "Tensor",
+                )
+            ],
         }
     )