fix warnings showing up with -Wall (#2692)

tests/gpu_tests.cpp

@@ -447,7 +447,7 @@ TEST_CASE("test gpu matmul") {
 TEST_CASE("test gpu validation") {
   // Run this test with Metal validation enabled
   // METAL_DEVICE_WRAPPER_TYPE=1 METAL_DEBUG_ERROR_MODE=0 ./tests/tests \
-  //     -tc="test metal validation" \
+  //     -tc="test metal validation"
 
   auto x = array({});
   eval(exp(x));

tests/ops_tests.cpp

@@ -3635,7 +3635,6 @@ TEST_CASE("test conv1d") {
           {1, 3, 2}),
       float16);
 
-  int kernel = 3;
   int stride = 1;
   int padding = 1;
 
@@ -3735,7 +3734,6 @@ TEST_CASE("test conv2d") {
        -0.26912728},
       {1, 2, 2, 2});
 
-  std::pair<int, int> kernel{2, 2};
   std::pair<int, int> stride{1, 1};
   std::pair<int, int> padding{0, 0};