Update a CNTK version on Travis (keras-team#10419)

.travis.yml

@@ -61,9 +61,9 @@ install:
 
   # install cntk
   - if [[ "$TRAVIS_PYTHON_VERSION" == "2.7" ]]; then
-      pip install https://cntk.ai/PythonWheel/CPU-Only/cntk-2.3.1-cp27-cp27mu-linux_x86_64.whl;
+      pip install https://cntk.ai/PythonWheel/CPU-Only/cntk-2.5.1-cp27-cp27mu-linux_x86_64.whl;
     elif [[ "$TRAVIS_PYTHON_VERSION" == "3.6" ]]; then
-      pip install https://cntk.ai/PythonWheel/CPU-Only/cntk-2.3.1-cp36-cp36m-linux_x86_64.whl;
+      pip install https://cntk.ai/PythonWheel/CPU-Only/cntk-2.5.1-cp36-cp36m-linux_x86_64.whl;
     fi
 
   # install pydot for visualization tests

keras/backend/cntk_backend.py

@@ -109,11 +109,12 @@ def in_test_phase(x, alt, training=None):
 
 
 def _convert_string_dtype(dtype):
-    # cntk only support float32 and float64
     if dtype == 'float32':
         return np.float32
     elif dtype == 'float64':
         return np.float64
+    elif dtype == 'float16':
+        return np.float16
     else:
         # cntk only running with float,
         # try to cast to float to run the model
@@ -125,10 +126,12 @@ def _convert_dtype_string(dtype):
         return 'float32'
     elif dtype == np.float64:
         return 'float64'
+    elif dtype == np.float16:
+        return 'float16'
     else:
         raise ValueError('CNTK Backend: Unsupported dtype: %s. '
-                         'CNTK only supports float32 and '
-                         'float64.' % dtype)
+                         'CNTK only supports float32, float64, and '
+                         'float16.' % dtype)
 
 
 def variable(value, dtype=None, name=None, constraint=None):

tests/keras/backend/backend_test.py

@@ -1767,11 +1767,7 @@ def test_set_floatx(self):
     def test_dtype(self):
         assert K.dtype(K.variable(1, dtype='float64')) == 'float64'
         assert K.dtype(K.variable(1, dtype='float32')) == 'float32'
-        if K.backend() == 'cntk':
-            with pytest.raises(ValueError):
-                K.variable(1, dtype='float16')
-        else:
-            assert K.dtype(K.variable(1, dtype='float16')) == 'float16'
+        assert K.dtype(K.variable(1, dtype='float16')) == 'float16'
 
     def test_variable_support_bool_dtype(self):
         # Github issue: 7819