Fix forwarding of initial_alpha and learning_rate in UMAP (#6417)

Previously these parameters weren't fully forwarded properly to `libcuml`.

Split out from #6316 (thanks Victor!)

Authors:
  - Jim Crist-Harif (https://github.com/jcrist)

Approvers:
  - Simon Adorf (https://github.com/csadorf)

URL: #6417

Author: jcrist

python/cuml/cuml/manifold/simpl_set.pyx

@@ -326,7 +326,8 @@ def simplicial_set_embedding(
 
     cdef UMAPParams* umap_params = new UMAPParams()
     umap_params.n_components = <int> n_components
-    umap_params.initial_alpha = <int> initial_alpha
+    umap_params.initial_alpha = <float> initial_alpha
+    umap_params.learning_rate = <float> initial_alpha
     umap_params.a = <float> a
     umap_params.b = <float> b
 

python/cuml/cuml/manifold/umap.pyx

@@ -470,6 +470,7 @@ class UMAP(UniversalBase,
             umap_params.n_components = <int> self.n_components
             umap_params.n_epochs = <int> self.n_epochs if self.n_epochs else 0
             umap_params.learning_rate = <float> self.learning_rate
+            umap_params.initial_alpha = <float> self.learning_rate
             umap_params.min_dist = <float> self.min_dist
             umap_params.spread = <float> self.spread
             umap_params.set_op_mix_ratio = <float> self.set_op_mix_ratio