Add torch.sparse.as_sparse_gradcheck decorator of gradcheck that allows gradcheck input function to receive and return sparse tensors (#107150)
Compared to #104848, this PR makes a step further: when the enable_sparse_support decorator is applied to `torch.autograd.gradcheck`, the resulting callable is equivalent to `torch.autograd.gradcheck` with an extra feature of supporting functions that can have input sparse tensors or/and can return sparse tensors.
At the same time, the underlying call to `torch.autograd.gradcheck` will operate on strided tensors only. This basically means that torch/autograd/gradcheck.py can be cleaned up by removing the code that deals with sparse tensors.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/107150
Approved by: https://github.com/albanD, https://github.com/amjames, https://github.com/cpuhrsch
ghstack dependencies: #107638, #107777
