Fix redundant functions' module names in auto-gen docs (#10664)

### Summary
This PR fixes the redundant function module names in auto-gen docs. Currently, the module name of functions (those which are not methods of a class) is repeated two times in documentation. [For example](https://keras.io/preprocessing/sequence/#pad_sequences):
> `keras.preprocessing.sequence.keras.preprocessing.sequence.pad_sequences`

which should be: `keras.preprocessing.sequence.pad_sequences`.
Further, this PR also removes from the docs the (wrong) module name of methods of classes like `ImageDataGenerator`. For example, [this is for `apply_transform`](https://keras.io/preprocessing/image/#apply_transform):

> `keras.preprocessing.image.apply_transform` 

which should simply be `apply_transform`, first because it is the method of a class (i.e. `ImageDataGenerator`) and second because the referenced module name is wrong (and this [has caused confusion](https://stackoverflow.com/q/51311062/2099607)).

All of this were caused by the fact that `clean_module_name()` was not called on the `function.__module__` in `render_function` (therefore this mostly affects the docs for `keras.preprocessing` and `keras.applications`). Plus, the way `autogen.py` works, the module name is repeated two times in the signature, but for the non-method functions it should be removed only once (that's why I have added a condition).

### Related Issues
#10658 
#10662 

### PR Overview

- [n] This PR requires new unit tests [y/n] (make sure tests are included)
- [n] This PR requires to update the documentation [y/n] (make sure the docs are up-to-date)
- [y] This PR is backwards compatible [y/n]
- [n] This PR changes the current API [y/n] (all API changes need to be approved by fchollet)

docs/autogen.py

@@ -589,7 +589,10 @@ def collect_class_methods(cls, methods):
 def render_function(function, method=True):
     subblocks = []
     signature = get_function_signature(function, method=method)
-    signature = signature.replace(function.__module__ + '.', '')
+    if method:
+        signature = signature.replace(clean_module_name(function.__module__) + '.', '')
+    else:
+        signature = signature.replace(clean_module_name(function.__module__) + '.', '', 1)
     subblocks.append('### ' + function.__name__ + '\n')
     subblocks.append(code_snippet(signature))
     docstring = function.__doc__