Fix minor Optional Dependency issues (#580)

* Add comment explaining the usage of ERROR_TYPES

* Fix tensorflow probability import edge case issue

alibi_detect/utils/frameworks.py

@@ -3,6 +3,7 @@
 
 try:
     import tensorflow as tf  # noqa
+    import tensorflow_probability as tfp  # noqa
     has_tensorflow = True
 except ImportError:
     has_tensorflow = False

alibi_detect/utils/missing_optional_dependency.py

@@ -18,6 +18,15 @@
 ))
 
 
+"""Mapping used to ensure correct pip install message is generated if a missing optional dependency is detected. This
+dict is used to control two behaviours:
+    1. When we import objects from missing dependencies we check that any `ModuleNotFoundError` or `ImportError`
+    corresponds to a missing optional dependency by checking the name of the missing dependency is in `ERROR_TYPES`. We
+    then map this name to the corresponding optional dependency bucket that will resolve the issue.
+    2. Some optional dependencies have multiple names such as `torch` and `pytorch`, instead of enforcing a single
+    naming convention across the whole code base we instead use `ERROR_TYPES` to capture both cases. This is done right
+    before the pip install message is issued as this is the most robust place to capture these differences.
+"""
 ERROR_TYPES = {
     "fbprophet": 'prophet',
     "holidays": 'prophet',