[trainer] improve code readability (#8903)

* [trainer] improve code

This PR:
- removes redundant code 
```
self.model = model if model is not None else None
```
and
```
self.model = model
```
are the same.

* separate attribute assignment from code logic - which simplifies things further.

* whitespace

src/transformers/trainer.py

@@ -241,11 +241,12 @@ class Trainer:
         self.hp_name = None
         if model is None and model_init is not None:
             model = self.call_model_init()
+
         # Model parallel
-        if not self.args.model_parallel:
+        if model is not None and not self.args.model_parallel:
-            self.model = model.to(args.device) if model is not None else None
+            model = model.to(args.device)
-        else:
+
-            self.model = model if model is not None else None
+        self.model = model
         default_collator = default_data_collator if tokenizer is None else DataCollatorWithPadding(tokenizer)
         self.data_collator = data_collator if data_collator is not None else default_collator
         self.train_dataset = train_dataset