Add Theano flag compute_test_value_opt.

doc/library/config.txt

@@ -630,6 +630,12 @@ import theano and print the config variable, as in:
           this Op
         - ``'raise'`` will raise an Exception
     
+.. attribute:: config.compute_test_value_opt
+
+   As ``compute_test_value``, but it is the value used during Theano
+   optimization phase. Theano user's do not need to use this. This is
+   to help debug shape error in Theano optimization.
+
 .. attribute:: config.exception_verbosity
 
     String Value: ``'low'``, ``'high'``.

theano/compile/debugmode.py

@@ -2118,7 +2118,7 @@ class _Maker(FunctionMaker):  # inheritance buys a few helper functions
             # optimize the fgraph
             compute_test_value_orig = theano.config.compute_test_value
             try:
-                theano.config.compute_test_value = "off"
+                theano.config.compute_test_value = theano.config.compute_test_value_opt
                 optimizer(fgraph)
 
                 theano.compile.function_module.insert_deepcopy(fgraph, inputs,

theano/compile/function_module.py

@@ -1018,7 +1018,7 @@ class FunctionMaker(object):
         compute_test_value_orig = theano.config.compute_test_value
         add_stack_trace_on_call = gof.Op.add_stack_trace_on_call
         try:
-            theano.config.compute_test_value = "off"
+            theano.config.compute_test_value = theano.config.compute_test_value_opt
             gof.Op.add_stack_trace_on_call = False
             start_optimizer = time.time()
             optimizer_profile = optimizer(fgraph)

theano/configdefaults.py

@@ -383,6 +383,13 @@ AddConfigVar('compute_test_value',
         in_c_key=False)
 
 
+AddConfigVar('compute_test_value_opt',
+             ("For debugging Theano optimization only."
+              " Same as compute_test_value, but is used"
+              " durint Theano optimizatoin"),
+             EnumStr('off', 'ignore', 'warn', 'raise'),
+             in_c_key=False)
+
 """Note to developers:
     Generally your exceptions should use an apply node's __str__
     method when exception_verbosity == 'low'. When exception_verbosity