tensor 广播机制
如果满足以下规则，则两个张量是“可广播的”：
-
每个张量至少有一个维度。
-
当开始比较各维度大小时，从最后的维度开始，维度大小必须相等、其中之一为 1，或者其中之一不存在。
比如：
>>> x=torch.empty(5,7,3)
>>> y=torch.empty(5,7,3)
# same shapes are always broadcastable (i.e. the above rules always hold)
>>> x=torch.empty((0,))
>>> y=torch.empty(2,2)
# x and y are not broadcastable, because x does not have at least 1 dimension
# can line up trailing dimensions
>>> x=torch.empty(5,3,4,1)
>>> y=torch.empty( 3,1,1)
# x and y are broadcastable.
# 1st trailing dimension: both have size 1
# 2nd trailing dimension: y has size 1
# 3rd trailing dimension: x size == y size
# 4th trailing dimension: y dimension doesn't exist
# but:
>>> x=torch.empty(5,2,4,1)
>>> y=torch.empty( 3,1,1)
# x and y are not broadcastable, because in the 3rd trailing dimension 2 != 3