MAINT: make ValDataLoaderIter able to be reset only when it is

acquired by the syntax of normal `iterator`

In `LRFinder.range_test()`, `val_iter` won't be reset after it runs
out of values, and it makes `LRFinder._validate()` failed to work
correctly after the first iteration of `range_test()`.

To fix it, we add a counter to count the times a `ValDataLoaderIter`
has run (i.e. times of `__next__()` is called). And reset it only
when its `__iter__()` is called. So that it won't be reset
automatically like the way `TrainDataLoaderIter` works.

See also davidtvs#59 and the docstring of `ValDataLoaderIter` for further
details.

torch_lr_finder/lr_finder.py

@@ -68,7 +68,39 @@ def __next__(self):
 
 
 class ValDataLoaderIter(DataLoaderIter):
-    pass
+    """This iterator will reset itself **only** when it is acquired by
+    the syntax of normal `iterator`. That is, this iterator just works
+    like a `torch.data.DataLoader`. If you want to restart it, you
+    should use it like:
+
+        ```
+        loader_iter = ValDataLoaderIter(data_loader)
+        for batch in loader_iter:
+            ...
+
+        # `loader_iter` should run out of values now, you can restart it by:
+        # 1. the way we use a `torch.data.DataLoader`
+        for batch in loader_iter:        # __iter__ is called implicitly
+            ...
+
+        # 2. passing it into `iter()` manually
+        loader_iter = iter(loader_iter)  # __iter__ is called by `iter()`
+        ```
+    """
+    def __init__(self, data_loader, auto_reset=True):
+        super().__init__(data_loader)
+        self.run_limit = len(self.data_loader)
+        self.run_counter = 0
+
+    def __iter__(self):
+        if self.run_counter >= self.run_limit:
+            self._iterator = iter(self.data_loader)
+            self.run_counter = 0
+        return self
+
+    def __next__(self):
+        self.run_counter += 1
+        return super(ValDataLoaderIter, self).__next__()
 
 
 class LRFinder(object):