Fix two warnings (#676)

- torch.nn.functional.sigmoid is deprecated in favor of torch.sigmoid.
- Clip cosh input in sechsq to avoid overflow.

thinc/backends/ops.py

@@ -1001,6 +1001,8 @@ def erf(self, X: FloatsType) -> FloatsType:
         return out
 
     def sechsq(self, X: FloatsType) -> FloatsType:
+        # Avoid overflow in cosh. Clipping at |20| has an error of 1.7e-17.
+        X = self.xp.clip(X, -20.0, 20.0)
         return (1 / self.xp.cosh(X)) ** 2
 
     def gelu_approx(self, X: FloatsType, inplace: bool = False) -> FloatsType:

thinc/tests/backends/test_ops.py

@@ -62,7 +62,7 @@ def torch_hard_swish_mobilenet(x):
         return torch.nn.functional.hardswish(x)
 
     def torch_sigmoid(x):
-        return torch.nn.functional.sigmoid(x)
+        return torch.sigmoid(x)
 
     # https://github.com/huggingface/transformers/blob/master/src/transformers/activations.py#L37
     def torch_gelu_approx(x):