Support for multiple val_dataloaders (#97)

* Added support for multiple validation dataloaders

* Fix typo in README.md

* Update trainer.py

* Add support for multiple dataloaders

* Rename dataloader_index to dataloader_i

* Added warning to check val_dataloaders

Added a warning to ensure that all val_dataloaders were DistributedSamplers if ddp is enabled

* Updated DistributedSampler warning

* Fixed typo

* Added multiple val_dataloaders

* Multiple val_dataloader test

* Update lightning_module_template.py

Added dataloader_i to validation_step parameters

* Update trainer.py

* Reverted template changes

* Create multi_val_module.py

* Update no_val_end_module.py

* New MultiValModel

* Rename MultiValModel to MultiValTestModel

* Revert to LightningTestModel

* Update test_models.py

* Update trainer.py

* Update test_models.py

* multiple val_dataloaders in test template

* Fixed flake8 warnings

* Update trainer.py

* Fix flake errors

* Fixed Flake8 errors

* Update lm_test_module.py

keep this test model with a single dataset for val

* Update trainer.py

* Update trainer.py

* Update trainer.py

* Update trainer.py

* Update trainer.py

* Update test_models.py

* Update trainer.py

* Update trainer.py

* Update trainer.py

* Update trainer.py

* Update trainer.py

* Update trainer.py

* Update RequiredTrainerInterface.md

* Update RequiredTrainerInterface.md

* Update test_models.py

* Update trainer.py

dont need the else clause, val_dataloader is either a list or none because of get_dataloaders()

* Update trainer.py

fixed flake errors

* Update trainer.py

examples/new_project_templates/lightning_module_template.py

@@ -105,7 +105,7 @@ class LightningTemplateModel(LightningModule):
         # can also return just a scalar instead of a dict (return loss_val)
         return output
 
-    def validation_step(self, data_batch, batch_i):
+    def validation_step(self, data_batch, batch_i, dataloader_i):
         """
         Lightning calls this inside the validation loop
         :param data_batch:
@@ -218,7 +218,7 @@ class LightningTemplateModel(LightningModule):
     @pl.data_loader
     def val_dataloader(self):
         print('val data loader called')
-        return self.__dataloader(train=False)
+        return [self.__dataloader(train=False) for i in range(2)]
 
     @pl.data_loader
     def test_dataloader(self):