julia: implement context.num_gpus (apache#16236)

MXNET-1427 #resolve

julia/src/MXNet.jl

@@ -74,7 +74,8 @@ export Executor,
 # context.jl
 export Context,
        cpu,
-       gpu
+       gpu,
+       num_gpus
 
 # model.jl
 export AbstractModel,

julia/src/context.jl

@@ -57,3 +57,14 @@ Get a GPU context with a specific id. The K GPUs on a node is typically numbered
 * `dev_id::Integer = 0` the GPU device id.
 """
 gpu(dev_id::Integer = 0) = Context(GPU, dev_id)
+
+"""
+    num_gpus()
+
+Query CUDA for the number of GPUs present.
+"""
+function num_gpus()
+  n = Ref{Cint}()
+  @mxcall :MXGetGPUCount (Ref{Cint},) n
+  n[]
+end

julia/test/unittest/context.jl

@@ -0,0 +1,34 @@
+# Licensed to the Apache Software Foundation (ASF) under one
+# or more contributor license agreements.  See the NOTICE file
+# distributed with this work for additional information
+# regarding copyright ownership.  The ASF licenses this file
+# to you under the Apache License, Version 2.0 (the
+# "License"); you may not use this file except in compliance
+# with the License.  You may obtain a copy of the License at
+#
+#   http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing,
+# software distributed under the License is distributed on an
+# "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+# KIND, either express or implied.  See the License for the
+# specific language governing permissions and limitations
+# under the License.
+
+module TestContext
+
+using MXNet
+using Test
+
+function test_num_gpus()
+  @info "Context::num_gpus"
+
+  @test num_gpus() >= 0
+end
+
+@testset "Context Test" begin
+  test_num_gpus()
+end
+
+
+end  # module TestContext

python/mxnet/context.py

@@ -276,6 +276,7 @@ def num_gpus():
     check_call(_LIB.MXGetGPUCount(ctypes.byref(count)))
     return count.value
 
+
 def gpu_memory_info(device_id=0):
     """Query CUDA for the free and total bytes of GPU global memory.
 
@@ -300,6 +301,7 @@ def gpu_memory_info(device_id=0):
     check_call(_LIB.MXGetGPUMemoryInformation64(dev_id, ctypes.byref(free), ctypes.byref(total)))
     return (free.value, total.value)
 
+
 def current_context():
     """Returns the current context.