Adding unbatching support for torch.sparse.Tensor

[DomInvivo]

This PR allows to fix the issues with Batch.from_data_list or Batch.get_example or Batch__getitem__ when using pytorch's sparse tensors, such as sparse_coo_tensor. See issue #7022

[codecov]

Codecov Report

Merging #7037 (56b8c59) into master (d3f4471) will decrease coverage by 0.01%.
The diff coverage is 90.90%.

❗ Current head 56b8c59 differs from pull request most recent head 097aa66. Consider uploading reports for the commit 097aa66 to get more accurate results

@@            Coverage Diff             @@
##           master    #7037      +/-   ##
==========================================
- Coverage   91.52%   91.52%   -0.01%     
==========================================
  Files         436      436              
  Lines       23832    23841       +9     
==========================================
+ Hits        21813    21821       +8     
- Misses       2019     2020       +1     

Impacted Files	Coverage Δ
torch_geometric/data/separate.py	98.27% <90.90%> (-1.73%)	⬇️

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