refactor(mge): use logsumexp in cross_entropy

GitOrigin-RevId: 4a14aabb94cc083762490ce009c69394db012e96

imperative/python/megengine/functional/loss.py

@@ -11,7 +11,7 @@ import numpy as np
 from ..core.tensor.utils import make_shape_tuple
 from ..tensor import Tensor
 from .elemwise import abs, equal, exp, log, maximum, pow, relu
-from .nn import indexing_one_hot, logsigmoid, logsoftmax
+from .nn import indexing_one_hot, logsigmoid, logsumexp
 from .tensor import where
 
 __all__ = [
@@ -191,9 +191,7 @@ def cross_entropy(
         )
 
     # Denominator of the softmax
-    offset = pred.detach().max(axis=axis, keepdims=True)
-    pred = pred - offset
-    down = log(exp(pred).sum(axis=axis, keepdims=True))
+    down = logsumexp(pred, axis=axis, keepdims=True)
 
     up = indexing_one_hot(pred, label, axis)
 

imperative/python/megengine/functional/nn.py

@@ -546,7 +546,7 @@ def logsumexp(
         [-0.5481  4.4519]
 
     """
-    max_value = max(inp, axis, keepdims=True)
+    max_value = max(inp.detach(), axis, keepdims=True)
     if keepdims:
         return max_value + log(sum(exp(inp - max_value), axis, keepdims))
     else: