https://pytorch.org/docs/stable/notes/broadcasting.html#broadcasting-semantics

https://pytorch.org/docs/stable/torch.html?highlight=matmul#torch.matmul

# 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