Don't crash when pydot isn't there.

theano/d3viz/formatting.py

@@ -14,7 +14,11 @@ from theano import gof
 from theano.compile.profilemode import ProfileMode
 from theano.compile import Function
 from theano.compile import builders
-from theano.printing import pd, pydot_imported, pydot_imported_msg
+from theano.printing import pydot_imported, pydot_imported_msg
+try:
+    from theano.printing import pd
+except ImportError:
+    pass
 
 
 class PyDotFormatter(object):