Minor cython fixes / cleanup (rapidsai#1072)

* Release GIL on C++ cython declarations
* Remove 'valid values for metric' mention from the compute_new_centroids docstring (since it doesn't take a metric parameter)
* Remove `is_c_cont` in favour of `input_validation.is_c_contiguous` in kmeans.pyx

Authors:
  - Ben Frederickson (https://github.com/benfred)

Approvers:
  - Corey J. Nolet (https://github.com/cjnolet)

URL: rapidsai#1072

python/pylibraft/pylibraft/cluster/cpp/kmeans.pxd

@@ -31,7 +31,7 @@ from pylibraft.common.handle cimport handle_t
 
 
 cdef extern from "raft_runtime/cluster/kmeans.hpp" \
-        namespace "raft::runtime::cluster::kmeans":
+        namespace "raft::runtime::cluster::kmeans" nogil:
 
     cdef void update_centroids(
         const handle_t& handle,

python/pylibraft/pylibraft/cluster/kmeans.pyx

@@ -46,12 +46,6 @@ from pylibraft.common.cpp.optional cimport optional
 from pylibraft.common.handle cimport handle_t
 
 
-def is_c_cont(cai, dt):
-    return "strides" not in cai or \
-        cai["strides"] is None or \
-        cai["strides"][1] == dt.itemsize
-
-
 @auto_sync_handle
 def compute_new_centroids(X,
                           centroids,
@@ -63,9 +57,6 @@ def compute_new_centroids(X,
     """
     Compute new centroids given an input matrix and existing centroids
 
-    Valid values for metric:
-        ["euclidean", "sqeuclidean"]
-
     Parameters
     ----------
 
@@ -167,9 +158,9 @@ def compute_new_centroids(X,
     handle = handle if handle is not None else Handle()
     cdef handle_t *h = <handle_t*><size_t>handle.getHandle()
 
-    x_c_contiguous = is_c_cont(x_cai, x_dt)
-    centroids_c_contiguous = is_c_cont(centroids_cai, centroids_dt)
-    new_centroids_c_contiguous = is_c_cont(new_centroids_cai, new_centroids_dt)
+    x_c_contiguous = is_c_contiguous(x_cai)
+    centroids_c_contiguous = is_c_contiguous(centroids_cai)
+    new_centroids_c_contiguous = is_c_contiguous(new_centroids_cai)
 
     if not x_c_contiguous or not centroids_c_contiguous \
             or not new_centroids_c_contiguous:
@@ -258,8 +249,8 @@ def cluster_cost(X, centroids, handle=None):
     handle = handle if handle is not None else Handle()
     cdef handle_t *h = <handle_t*><size_t>handle.getHandle()
 
-    x_c_contiguous = is_c_cont(x_cai, x_dt)
-    centroids_c_contiguous = is_c_cont(centroids_cai, centroids_dt)
+    x_c_contiguous = is_c_contiguous(x_cai)
+    centroids_c_contiguous = is_c_contiguous(centroids_cai)
 
     if not x_c_contiguous or not centroids_c_contiguous:
         raise ValueError("Inputs must all be c contiguous")

python/pylibraft/pylibraft/distance/fused_l2_nn.pyx

@@ -33,7 +33,7 @@ from pylibraft.common.handle cimport handle_t
 
 
 cdef extern from "raft_runtime/distance/fused_l2_nn.hpp" \
-        namespace "raft::runtime::distance":
+        namespace "raft::runtime::distance" nogil:
 
     void fused_l2_nn_min_arg(
         const handle_t &handle,

python/pylibraft/pylibraft/distance/pairwise_distance.pyx

@@ -35,7 +35,7 @@ from pylibraft.common import cai_wrapper, device_ndarray
 
 
 cdef extern from "raft_runtime/distance/pairwise_distance.hpp" \
-        namespace "raft::runtime::distance":
+        namespace "raft::runtime::distance" nogil:
 
     cdef void pairwise_distance(const handle_t &handle,
                                 float *x,

python/pylibraft/pylibraft/random/rmat_rectangular_generator.pyx

@@ -33,7 +33,7 @@ from pylibraft.random.cpp.rng_state cimport RngState
 
 
 cdef extern from "raft_runtime/random/rmat_rectangular_generator.hpp" \
-        namespace "raft::runtime::random":
+        namespace "raft::runtime::random" nogil:
 
     cdef void rmat_rectangular_gen(const handle_t &handle,
                                    int* out,