feat(wandb): improve logging of config parameters

[borisdayma]

Following changes have been implemented:

• log slices (example with Learner.fit(1, lr=slice(0.05))
  
• improve logging of config parameters (try to log all at once and if it fails, one by one)
• handle cases where we don't succeed in logging prediction samples (either when creating dataloader or when performing predictions)
• fix for bug with dataframes as reported in forums

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[sgugger]

All looking good, thanks!