[refactor] Extend WandbLogger to log config variables, entity and kwargs

[ayulockin]

1. I was going through the WandbLogger class that was recently added to this library. Since MMF is a config-first framework, it would be great to log the config file to keep track of what's going in the model training/evaluation process.

   The effect of this small change can be seen through the before and after screenshots. One can compare one experiment from another by comparing the config.

   Before:
   

   After:
   

2. I have also added another argument to pass entity to wandb.init. Explicit mention of this will help users who are using a W&B Teams account.

3. I also saw the **init_kwargs argument in the WandbLogger (logger.py) class but the same was not passed in the LogisticsCallback (logistics.py) class. I might not be using the command line correctly to pass the kwargs arguments. If that's the case, I will revert back the change and would appreciate it if you can let me know the correct way of doing it.

PS: I am learning about this library and will be using it for personal work. Since I am also affiliated with Weights and Biases, I will be creating some PRs to add functionalities to the logger that might be useful for everyone. :)