Lazy load wandb

[cgarciae]

Currently wandb is imported at the top-level making it a strict dependency, to avoid cluttering too much the dependency space, this PR add wandb as an optional dependecy which the user could install via the all extras tag:

pip install 'elegy[all]'

Additionally, this fixes the current error on wandb CI error on master.

[codecov-commenter]

Codecov Report

Merging #228 (979b5f5) into master (bc2483c) will increase coverage by 0.35%.
The diff coverage is 44.44%.

@@            Coverage Diff             @@
##           master     #228      +/-   ##
==========================================
+ Coverage   67.25%   67.60%   +0.35%     
==========================================
  Files          31       31              
  Lines        2446     2479      +33     
==========================================
+ Hits         1645     1676      +31     
- Misses        801      803       +2     

Impacted Files	Coverage Δ
elegy/callbacks/wandb_callback.py	11.21% <0.00%> (-1.05%)	⬇️
elegy/model/model_core.py	83.77% <100.00%> (ø)
elegy/model/model_base.py	84.86% <0.00%> (+0.08%)	⬆️
elegy/utils.py	37.30% <0.00%> (+0.32%)	⬆️
elegy/data/array_adapter.py	88.88% <0.00%> (+0.36%)	⬆️
elegy/data/dataset.py	84.61% <0.00%> (+0.55%)	⬆️
elegy/data/list_adapter.py	90.47% <0.00%> (+0.73%)	⬆️
elegy/data/tf_dataset_adapter.py	86.00% <0.00%> (+0.89%)	⬆️
elegy/data/data_handler.py	72.41% <0.00%> (+1.32%)	⬆️
elegy/data/torch_dataloader_adapter.py	75.47% <0.00%> (+1.47%)	⬆️
... and 2 more

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