[minor][wandb] Save task arguments as separate config values (#4484)

* [minor][wandb] Save task arguments as separate config values

Convenience so that we can do UI filters in wandb on values for task arguments.

```
>>> from parlai.core.logs import WandbLogger
>>> opt= {"wandb_name": "test_3", "wandb_project": "blah_2", "model_file": "dummy_mf", "task": "blah:x=y:a=2"}
>>> dummy = WandbLogger(opt)
>>> dummy.run.config
{'wandb_name': 'test_3', 'wandb_project': 'blah_2', 'model_file': 'dummy_mf', 'task': 'blah:x=y:a=2', 'x': 'y', 'a': '2'}

* Update from running in practice

* update again from running in practice lol (but it works now and flows end-to-end to my fairwandb)

parlai/core/logs.py

@@ -181,14 +181,25 @@ def __init__(self, opt: Opt, model=None):
             entity=opt.get('wandb_entity'),
             reinit=True,  # in case of preemption
             resume=True,  # requeued runs should be treated as single run
+            allow_val_change=True,
         )
         # suppress wandb's output
         logging.getLogger("wandb").setLevel(logging.ERROR)
 
+        def set_config_value(self, key, value):
+            if self.run.config.get(key, None) != None:
+                setattr(self.run.config, k, v)
+
         if not self.run.resumed:
             for key, value in opt.items():
                 if value is None or isinstance(value, (str, numbers.Number, tuple)):
-                    setattr(self.run.config, key, value)
+                    set_config_value(self, key, value)
+                if key == "task":  # For ags specified in the task argument
+                    maybe_task_opts = value.split(":")
+                    for task_opt in maybe_task_opts:
+                        if len(task_opt.split("=")) == 2:
+                            k, v = task_opt.split("=")
+                            set_config_value(self, k, v)
         if model is not None:
             self.run.watch(model)