Embedding Projector: reduce knn computation (#6321)

## Motivation for features / changes
for t-SNE max number of knn neighbors is 3 * max_perplexity (3 * 100 =
300). For umap max number of neighbors is 100.
let's just compute 300 knn neighbors so we can slice the result each
time when we tweak perplexity for tSNE.

## Technical description of changes
When computing KNN, use an upper threshold of 300 so any future
computation with same number of points will be cached.


## Screenshots of UI changes

N/A

## Detailed steps to verify changes work correctly (as executed by you)
1. Launch local app
2. Open t-SNE
3. adjust to Perplexity > 25
4. verify knn isn't computed a second time

## Alternate designs / implementations considered

tensorboard/plugins/projector/vz_projector/data.ts

@@ -97,6 +97,8 @@ export const PCA_SAMPLE_DIM = 200;
 const NUM_PCA_COMPONENTS = 10;
 /** Id of message box used for umap optimization progress bar. */
 const UMAP_MSG_ID = 'umap-optimization';
+/** Minimum KNN neighbors threshold */
+const MIN_NUM_KNN_NEIGHBORS = 300;
 /**
  * Reserved metadata attributes used for sequence information
  * NOTE: Use "__seq_next__" as "__next__" is deprecated.
@@ -474,16 +476,26 @@ export class DataSet {
       );
     } else {
       const knnGpuEnabled = (await util.hasWebGLSupport()) && !IS_FIREFOX;
+      const numKnnNeighborsToCompute = Math.max(
+        nNeighbors,
+        MIN_NUM_KNN_NEIGHBORS
+      );
       const result = await (knnGpuEnabled
-        ? knn.findKNNGPUCosDistNorm(data, nNeighbors, (d) => d.vector)
+        ? knn.findKNNGPUCosDistNorm(
+            data,
+            numKnnNeighborsToCompute,
+            (d) => d.vector
+          )
         : knn.findKNN(
             data,
-            nNeighbors,
+            numKnnNeighborsToCompute,
             (d) => d.vector,
             (a, b) => vector.cosDistNorm(a, b)
           ));
       this.nearest = result;
-      return Promise.resolve(result);
+      return Promise.resolve(
+        result.map((neighbors) => neighbors.slice(0, nNeighbors))
+      );
     }
   }
   /* Perturb TSNE and update dataset point coordinates. */