fix nonzero upper bound (#393)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/393

If nonzero's input has `n` dimensions, then the resulting output tensor is of size `(z, n)`, where `z` is the total number of non-zero elements in the input tensor. Therefore, an upper bound for out's shape is [input.numel(), input.dim()]

Reviewed By: ydwu4

Differential Revision: D49342476

fbshipit-source-id: ac70fd83090cc80c93315b9fc5be830afb6207df