Concat broadcast rewrite to handle more cases

Found during investigation for issue #3023

Issue is in find_concat_multibroadcasts of simplify_reshapes.
What we matched is:

param0: lens = [1, 1, 1, 1, 64], strides = [1, 1, 60, 64, 194]
param1: lens = [1, 1, 60, 64, 192], strides = [737280, 737280, 12288, 192 ,1]
mb0: multibroadcast(param0, lens = [1, 12, 60, 64, 64])
mb1: multibroadcast(param1, lens = [1, 12, 60, 64, 192])
concat(mb0, mb1, axis = 4)

It currently tries to move the concat before the multibroadcast to:

param0: lens = [1, 1, 1, 1, 64], strides = [1, 1, 60, 64, 194]
param1: lens = [1, 1, 60, 64, 192], strides = [737280, 737280, 12288, 192 ,1]
con0: concat(param0, param1, axis = 4)
multibroadcast(con0, lens = [1, 12, 60, 64, 256])

This fails because concat does not work if the dimensions other than axis to concat over are not the same.

• We should be able to rewrite concat to handle this case also.

Start at find_concat_multibroadcasts in simplify_reshapes and reproduce the two multibroadcast inputs and axis-4 concat shown in the issue. Trace why the current concat-before-multibroadcast rewrite rejects unequal non-concat dimensions; done means the concat broadcast rewrite handles this case without violating concat’s dimension requirements.