fix for docs and other minor fixes

doc/library/config.txt

@@ -255,6 +255,21 @@ import theano and print the config variable, as in:
     not predictable, so if you are close to the peak memory usage, trying both
     could give you a small gain.
 
+.. attribute:: check_stack_trace
+
+    String value, either ``off``, ``log``, ``warn``, ``raise``
+
+    Default: ``off``
+
+    This is a flag for checking the stack trace during the optimization process.
+    If :attr:`check_stack_trace` is set to ``off``, no check is performed on the
+    stack trace. If :attr:`check_stack_trace` is set to ``log`` or ``warn``, a
+    dummy stack trace is inserted that indicates which optimization inserted the
+    variable that had an empty stack trace but, in ``warn`` a warning is also
+    printed.
+    If :attr:`check_stack_trace` is set to ``raise``, an exception is raised if a
+    stack trace is missing.
+
 .. attribute:: openmp
 
     Bool value: either ``True`` or ``False``

theano/compile/mode.py

@@ -227,13 +227,14 @@ optdb.register('merge3', gof.MergeOptimizer(),
                100, 'fast_run', 'merge')
 
 if theano.config.check_stack_trace in ['raise', 'warn', 'log']:
-    tags = ('fast_run', 'fast_compile')
+    _tags = ('fast_run', 'fast_compile')
 
 if theano.config.check_stack_trace == 'off':
-    tags = ()
+    _tags = ()
 
 optdb.register('CheckStackTrace',
-               gof.CheckStackTraceOptimization(), -1, *tags)
+               gof.CheckStackTraceOptimization(), -1, *_tags)
+del _tags
 
 
 class Mode(object):

theano/configdefaults.py

@@ -1509,8 +1509,10 @@ AddConfigVar('check_stack_trace',
              "A flag for checking the stack trace during the optimization process. "
              "default (off): does not check the stack trace of any optimization "
              "log: inserts a dummy stack trace that identifies the optimization"
-             "that inserted that variable."
-             "warn: prints a warning if a stack trace is missing"
+             "that inserted the variable that had an empty stack trace."
+             "warn: prints a warning if a stack trace is missing and also a dummy"
+             "stack trace is inserted that indicates which optimization inserted"
+             "the variable that had an empty stack trace."
              "raise: raises an exception if a stack trace is missing",
              EnumStr('off', 'log', 'warn', 'raise'),
              in_c_key=False)

theano/gof/opt.py

@@ -3047,8 +3047,8 @@ class CheckStrackTraceFeature(object):
         # theano.config.check_stack_trace is not off but we also double check here.
         if theano.config.check_stack_trace != 'off' and not check_stack_trace(fgraph, 'all'):
             if theano.config.check_stack_trace == 'raise':
-                    raise NotImplementedError(
-                        'Empty stack trace! The optimization that inserted this variable is' + str(reason))
+                    raise AssertionError(
+                        'Empty stack trace! The optimization that inserted this variable is ' + str(reason))
             elif theano.config.check_stack_trace in ['log', 'warn']:
                 apply_nodes_to_check = fgraph.apply_nodes
                 for node in apply_nodes_to_check: