Add support for sparse CSR tensors in gcn_norm

[rusty1s]

No description provided.

[codecov]

Codecov Report

Merging #6957 (6700d4a) into master (9bda455) will decrease coverage by 0.03%.
The diff coverage is 90.69%.

@@            Coverage Diff             @@
##           master    #6957      +/-   ##
==========================================
- Coverage   91.45%   91.43%   -0.03%     
==========================================
  Files         429      429              
  Lines       23357    23391      +34     
==========================================
+ Hits        21361    21387      +26     
- Misses       1996     2004       +8     

Impacted Files	Coverage Δ
torch_geometric/utils/sparse.py	86.30% <70.83%> (-6.81%)	⬇️
torch_geometric/nn/conv/cluster_gcn_conv.py	98.43% <96.29%> (-1.57%)	⬇️
torch_geometric/nn/conv/appnp.py	100.00% <100.00%> (ø)
torch_geometric/nn/conv/fa_conv.py	95.06% <100.00%> (+0.06%)	⬆️
torch_geometric/nn/conv/gat_conv.py	100.00% <100.00%> (ø)
torch_geometric/nn/conv/gcn_conv.py	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

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