Add dnn.enabled=no_check to speed up import

doc/library/config.txt

@@ -693,6 +693,8 @@ import theano and print the config variable, as in:
 
     If ``'False'``, do not use cuDNN or check if it is available.
 
+    If ``'no_check'``, assume present and the version between heasers and linked match (so less compilation at context init)",
+
 .. attribute:: config.conv.assert_shape
 
     If True, AbstractConv* ops will verify that user-provided

theano/configdefaults.py

@@ -414,8 +414,9 @@ AddConfigVar('dnn.enabled',
              "'auto', use cuDNN if available, but silently fall back"
              " to not using it if not present."
              " If True and cuDNN can not be used, raise an error."
-             " If False, disable cudnn",
-             EnumStr("auto", "True", "False"),
+             " If False, disable cudnn even if present."
+             " If no_check, assume present and the version between heasers and linked match (so less compilation at context init)",
+             EnumStr("auto", "True", "False", "no_check"),
              in_c_key=False)
 
 # This flag determines whether or not to raise error/warning message if

theano/gpuarray/dnn.py

@@ -152,6 +152,9 @@ def dnn_present():
         dnn_present.avail = False
         return False
 
+    if config.dnn.enabled == "no_check":
+        dnn_present.avail, dnn_present.msg = True, "presence check disabled by dnn.enabled flag"
+    else:
         dnn_present.avail, dnn_present.msg = _dnn_check_compile()
     if dnn_present.avail:
         dnn_present.avail, dnn_present.msg = _dnn_check_version()

theano/sandbox/cuda/__init__.py

@@ -282,6 +282,8 @@ def dnn_available():
     if dnn_available.avail is None and not cuda_available:
         dnn_available.msg = "CUDA not available"
         dnn_available.avail = False
+    elif config.dnn.enabled == "no_check":
+        raise RuntimeException("The old gpu back-end do not support the flag dnn.enabled=no_check")
     elif dnn_available.avail is None:
         dev = active_device_number()
         if device_properties(dev)['major'] < 3: