Add way to specify the allow_gc flags on individual scan instances.

theano/scan_module/scan.py

@@ -74,7 +74,8 @@ def scan(fn,
          go_backwards=False,
          mode=None,
          name=None,
-         profile=False):
+         profile=False,
+         allow_gc=None):
     """
     This function constructs and applies a Scan op to the provided
     arguments.
@@ -309,6 +310,10 @@ def scan(fn,
         inner graph with the new cvm linker ( with default modes,
         other linkers this argument is useless)
 
+    :param allow_gc:
+        Set the value of allow gc for the internal graph of scan.  If
+        set to None, this will use the value of config.scan.allow_gc.
+
     :rtype: tuple
     :return: tuple of the form (outputs, updates); ``outputs`` is either a
              Theano variable or a list of Theano variables representing the
@@ -981,7 +986,7 @@ def scan(fn,
     info['as_while'] = as_while
     info['profile'] = profile
 
-    local_op = scan_op.Scan(inner_inputs, new_outs, info)
+    local_op = scan_op.Scan(inner_inputs, new_outs, info, allow_gc=allow_gc)
 
     ##
     ### Step 8. Compute the outputs using the scan op