fix concat and split tensor inference function (#21382)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/21382
Concat tensor inference function was not handling correctly the case where axis argument points to the last dimension so input tensors don't need to have the same number of dimensions.
Split tensor inference function was not handling correctly the case where split information is provided as the second input tensor rather than as an argument.
Reviewed By: mdschatz
Differential Revision: D15633148
fbshipit-source-id: d566af44dc882457ee9efe83d2461b28408c2c5d
