Replace cpdef variables with cdef variables. (rapidsai#5145)

This PR fixes some warnings in the cuml Python build that will become errors in Cython 3. Examples follow:
```
cuml/python/cuml/decomposition/pca.pyx:334:14: cpdef variables will not be supported in Cython 3; currently they are no different from cdef variables
cuml/python/cuml/decomposition/pca.pyx:587:14: cpdef variables will not be supported in Cython 3; currently they are no different from cdef variables
cuml/python/cuml/decomposition/pca.pyx:683:14: cpdef variables will not be supported in Cython 3; currently they are no different from cdef variables
cuml/python/cuml/decomposition/tsvd.pyx:282:14: cpdef variables will not be supported in Cython 3; currently they are no different from cdef variables
cuml/python/cuml/decomposition/tsvd.pyx:398:14: cpdef variables will not be supported in Cython 3; currently they are no different from cdef variables
cuml/python/cuml/decomposition/tsvd.pyx:450:14: cpdef variables will not be supported in Cython 3; currently they are no different from cdef variables
cuml/python/cuml/decomposition/pca_mg.pyx:94:14: cpdef variables will not be supported in Cython 3; currently they are no different from cdef variables
cuml/python/cuml/decomposition/tsvd_mg.pyx:77:14: cpdef variables will not be supported in Cython 3; currently they are no different from cdef variables
cuml/python/cuml/fil/fil.pyx:82:10: cpdef variables will not be supported in Cython 3; currently they are no different from cdef variables
cuml/python/cuml/fil/fil.pyx:83:10: cpdef variables will not be supported in Cython 3; currently they are no different from cdef variables
```

Authors:
  - Bradley Dice (https://github.com/bdice)

Approvers:
  - William Hicks (https://github.com/wphicks)
  - Carl Simon Adorf (https://github.com/csadorf)

URL: rapidsai#5145

python/cuml/decomposition/pca.pyx

@@ -331,7 +331,7 @@ class PCA(UniversalBase,
         return algo_map[algorithm]
 
     def _build_params(self, n_rows, n_cols):
-        cpdef paramsPCA *params = new paramsPCA()
+        cdef paramsPCA *params = new paramsPCA()
         params.n_components = self.n_components_
         params.n_rows = n_rows
         params.n_cols = n_cols
@@ -582,7 +582,7 @@ class PCA(UniversalBase,
         cdef uintptr_t _trans_input_ptr = X_m.ptr
 
         # todo: check n_cols and dtype
-        cpdef paramsPCA params
+        cdef paramsPCA params
         params.n_components = self.n_components_
         params.n_rows = n_rows
         params.n_cols = self.n_features_in_
@@ -678,7 +678,7 @@ class PCA(UniversalBase,
         cdef uintptr_t input_ptr = X_m.ptr
 
         # todo: check dtype
-        cpdef paramsPCA params
+        cdef paramsPCA params
         params.n_components = self.n_components_
         params.n_rows = n_rows
         params.n_cols = n_cols

python/cuml/decomposition/pca_mg.pyx

@@ -91,7 +91,7 @@ class PCAMG(BaseDecompositionMG, PCA):
         return algo_map[algorithm]
 
     def _build_params(self, n_rows, n_cols):
-        cpdef paramsPCAMG *params = new paramsPCAMG()
+        cdef paramsPCAMG *params = new paramsPCAMG()
         params.n_components = self.n_components_
         params.n_rows = n_rows
         params.n_cols = n_cols

python/cuml/decomposition/tsvd.pyx

@@ -279,7 +279,7 @@ class TruncatedSVD(UniversalBase,
         return algo_map[algorithm]
 
     def _build_params(self, n_rows, n_cols):
-        cpdef paramsTSVD *params = new paramsTSVD()
+        cdef paramsTSVD *params = new paramsTSVD()
         params.n_components = self.n_components
         params.n_rows = n_rows
         params.n_cols = n_cols
@@ -395,7 +395,7 @@ class TruncatedSVD(UniversalBase,
                                 convert_to_dtype=(dtype if convert_dtype
                                                   else None))
 
-        cpdef paramsTSVD params
+        cdef paramsTSVD params
         params.n_components = self.n_components
         params.n_rows = n_rows
         params.n_cols = self.n_features_in_
@@ -447,7 +447,7 @@ class TruncatedSVD(UniversalBase,
                                                   else None),
                                 check_cols=self.n_features_in_)
 
-        cpdef paramsTSVD params
+        cdef paramsTSVD params
         params.n_components = self.n_components
         params.n_rows = n_rows
         params.n_cols = self.n_features_in_

python/cuml/decomposition/tsvd_mg.pyx

@@ -74,7 +74,7 @@ class TSVDMG(BaseDecompositionMG, TruncatedSVD):
         super(TSVDMG, self).__init__(**kwargs)
 
     def _build_params(self, n_rows, n_cols):
-        cpdef paramsTSVDMG *params = new paramsTSVDMG()
+        cdef paramsTSVDMG *params = new paramsTSVDMG()
         params.n_components = self.n_components_
         params.n_rows = n_rows
         params.n_cols = n_cols

python/cuml/fil/fil.pyx

@@ -79,8 +79,8 @@ cdef class TreeliteModel():
     handle : ModelHandle
         Opaque pointer to Treelite model
     """
-    cpdef ModelHandle handle
-    cpdef bool owns_handle
+    cdef ModelHandle handle
+    cdef bool owns_handle
 
     def __cinit__(self, owns_handle=True):
         """If owns_handle is True, free the handle's model in destructor.