Fix DataParallel validation forward signatures (#47)

* Fix: DataParallel validation forward signatures

* Update: generalize forward_fn selection

* nit: space

Author: KSGulin

src/transformers/trainer.py

@@ -2435,11 +2435,12 @@ def evaluation_loop(
 
         observed_num_examples = 0
         # Main evaluation loop
+        module_forward_fn = model.module.forward if isinstance(model, nn.DataParallel) else model.forward
         for step, inputs in enumerate(dataloader):
             inputs = {
                 k: inputs[k]
                 for k in inputs
-                if k in list(inspect.signature(model.forward).parameters.keys())
+                if k in list(inspect.signature(module_forward_fn).parameters.keys())
             }
 
             # Update the observed num examples