Propagate symmetries through broadcasts

[wiwu2390]

1. Propagate symmetries through broadcasts by taking the product of the existing symmetry group on the original indices with the full permutation group on the broadcast indices.
2. Apply this logic to auto-broadcast arguments of pointwise ops

Example:

A = we.random.randn(n, n, n, n)
A = we.symmetrize(A, group=we.PermutationGroup.symmetric(4,axes=(0,1,2,3)))
B = we.random.randn(n, n)
B = we.symmetrize(B, group=we.PermutationGroup.symmetric(2,axes=(0,1)))
B_bc = we.broadcast_to(B, A.shape)   # Should have symmetric_axes=((0,1), (2,3))
C = A * B  # Should have symmetric_axes=((0,1), (2,3))