Add HeteroData support to RemoveIsolatedNodes

[Padarn]

This PR adds HeteroData support to the RemoveIsolatedNodes transform, closing #4013.

This is achieved by looping through the different node types and removing them one at a time.

[codecov]

Codecov Report

Merging #4479 (8442a1d) into master (3f0019f) will decrease coverage by 0.04%.
The diff coverage is 90.74%.

❗ Current head 8442a1d differs from pull request most recent head a38c592. Consider uploading reports for the commit a38c592 to get more accurate results

@@            Coverage Diff             @@
##           master    #4479      +/-   ##
==========================================
- Coverage   82.69%   82.64%   -0.05%     
==========================================
  Files         313      313              
  Lines       16240    16303      +63     
==========================================
+ Hits        13429    13474      +45     
- Misses       2811     2829      +18     

Impacted Files	Coverage Δ
...orch_geometric/transforms/remove_isolated_nodes.py	90.47% <90.74%> (-4.27%)	⬇️
torch_geometric/data/hetero_data.py	93.10% <0.00%> (-2.30%)	⬇️
torch_geometric/data/data.py	90.70% <0.00%> (-1.41%)	⬇️
torch_geometric/loader/link_neighbor_loader.py	89.52% <0.00%> (-0.80%)	⬇️
torch_geometric/nn/conv/sage_conv.py	100.00% <0.00%> (ø)
torch_geometric/nn/conv/message_passing.py	99.43% <0.00%> (ø)

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