Fixed val interval (#405)

* added fixed frequency val batch check

* added fixed frequency val batch check

* Finished IterableDataset support

* flake8

* flake8

* flake8

docs/Trainer/Validation loop.md

@@ -43,13 +43,20 @@ trainer = Trainer(test_percent_check=0.1)
 
 ---
 ####  Set validation check frequency within 1 training epoch
-For large datasets it's often desirable to check validation multiple times within a training loop
+For large datasets it's often desirable to check validation multiple times within a training loop.
+Pass in a float to check that often within 1 training epoch.
+Pass in an int k to check every k training batches. Must use an int if using 
+an IterableDataset.
+ 
 ``` {.python}
 # DEFAULT
 trainer = Trainer(val_check_interval=0.95)
 
 # check every .25 of an epoch 
 trainer = Trainer(val_check_interval=0.25)
+
+# check every 100 train batches (ie: for IterableDatasets or fixed frequency)
+trainer = Trainer(val_check_interval=100)
 ```
 
 ---