Add Cython definitions for device_uvector (#720)

The new cuDF [join APIs](rapidsai/cudf#7454) will return gathermaps of type `device_uvector<size_type>>`, so we need Cython definitions for it in RMM.

Authors:
  - Ashwin Srinath (@shwina)

Approvers:
  - Keith Kraus (@kkraus14)

URL: #720

Author: shwina

python/rmm/_lib/device_uvector.pxd

@@ -0,0 +1,26 @@
+# Copyright (c) 2021, NVIDIA CORPORATION.
+
+from rmm._lib.device_buffer cimport device_buffer
+from rmm._lib.cuda_stream_view cimport cuda_stream_view
+from rmm._lib.memory_resource cimport device_memory_resource
+
+
+cdef extern from "rmm/device_buffer.hpp" namespace "rmm" nogil:
+    cdef cppclass device_uvector[T]:
+        device_uvector(size_t size, cuda_stream_view  stream) except +
+        T* element_ptr(size_t index)
+        void set_element(size_t element_index, const T& v, cuda_stream_view s)
+        void set_element_async(
+            size_t element_index,
+            const T& v,
+            cuda_stream_view s
+        ) except +
+        T front_element(cuda_stream_view s) except +
+        T back_element(cuda_stream_view s) except +
+        void resize(size_t new_size, cuda_stream_view stream) except +
+        void shrink_to_fit(cuda_stream_view stream) except +
+        device_buffer release()
+        size_t capacity()
+        T* data()
+        size_t size()
+        device_memory_resource* memory_resource()