add nested tensor matmul support (#81957)
There was a discussion on whether letting nested tensor `reshape` support collapsing and splitting dimension 0. The conclusion was to make reshape simple, so we need a tweaked `matmul`, which only supports 3+ dimension nonbroadcast case, i.e. a generalized `bmm`.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/81957
Approved by: https://github.com/jbschlosser
