Lazy load data.lightning

[rusty1s]

Most import time when calling import torch_geometric is spent on import pytorch_lightning. This PR moves the import of PL to its own sub-module - leading to a better experiencing when using raw PyG.

[codecov]

Codecov Report

Merging #6140 (620081c) into master (93289a4) will increase coverage by 0.00%.
The diff coverage is 92.59%.

@@           Coverage Diff           @@
##           master    #6140   +/-   ##
=======================================
  Coverage   84.35%   84.35%           
=======================================
  Files         363      365    +2     
  Lines       20483    20502   +19     
=======================================
+ Hits        17278    17295   +17     
- Misses       3205     3207    +2     

Impacted Files	Coverage Δ
torch_geometric/data/lightning/datamodule.py	55.90% <ø> (ø)
torch_geometric/lazy_loader.py	88.88% <88.88%> (ø)
torch_geometric/data/__init__.py	100.00% <100.00%> (ø)
torch_geometric/data/lightning/__init__.py	100.00% <100.00%> (ø)
torch_geometric/graphgym/imports.py	100.00% <100.00%> (ø)
torch_geometric/graphgym/logger.py	84.45% <100.00%> (-0.16%)	⬇️
torch_geometric/graphgym/train.py	100.00% <100.00%> (ø)

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