Cuda support for the mnist training. (#277)

* Cuda support for the mnist training.

* min/max fix + testing.

* Add the argmin/argmax tests.

* More cuda support for argmin/argmax.

* Cuda kernels for argmin and argmax.

candle-core/src/cpu_backend.rs

@@ -244,7 +244,7 @@ impl ReduceIndex {
                             val = s
                         }
                     }
-                    dst[unstr_index] = g(val, acc)
+                    dst_to_set[unstr_index] = g(val, acc)
                 }
             }
         }