Separate in-edge and out-edge contribution types (#491)

Summary:
This change is motivated by the behavior of loops, but applies more
generally to edges. Previously, a loop would induce two contributions
with the same contributor, but possibly different weights: one with the
to-weight of the edge and one with the fro-weight. For one, this is
annoying to downstream clients, who would like to use the contributor as
a superkey. But it is also somewhat strange that a single contributor
could have two different weights.

The same applies to edges in general: every edge induces two
contributions with the same contributor, of type `NEIGHBOR`.

As of this commit, we replace `NEIGHBOR` with `IN_EDGE` and `OUT_EDGE`,
one of each induced by each edge. This has the effect that a contributor
maps to at most one contribution.

Test Plan:
Existing unit tests updated.

wchargin-branch: separate-in-out-edge-contributions

src/core/attribution/graphToMarkovChain.js

@@ -1,18 +1,13 @@
 // @flow
 
-import {
-  type Edge,
-  type Graph,
-  type Neighbor,
-  type NodeAddressT,
-  NodeAddress,
-} from "../graph";
+import {type Edge, type Graph, type NodeAddressT, NodeAddress} from "../graph";
 import type {Distribution, SparseMarkovChain} from "./markovChain";
 
 export type Probability = number;
 export type Contributor =
   | {|+type: "SYNTHETIC_LOOP"|}
-  | {|+type: "NEIGHBOR", +neighbor: Neighbor|};
+  | {|+type: "IN_EDGE", +edge: Edge|}
+  | {|+type: "OUT_EDGE", +edge: Edge|};
 export type Contribution = {|
   +contributor: Contributor,
   // This `weight` is a conditional probability: given that you're at
@@ -28,8 +23,10 @@ export function contributorSource(
   switch (contributor.type) {
     case "SYNTHETIC_LOOP":
       return target;
-    case "NEIGHBOR":
-      return contributor.neighbor.node;
+    case "IN_EDGE":
+      return contributor.edge.src;
+    case "OUT_EDGE":
+      return contributor.edge.dst;
     default:
       throw new Error((contributor.type: empty));
   }
@@ -104,11 +101,11 @@ export function createContributions(
     const {toWeight, froWeight} = edgeWeight(edge);
     const {src, dst} = edge;
     processContribution(dst, {
-      contributor: {type: "NEIGHBOR", neighbor: {node: src, edge}},
+      contributor: {type: "IN_EDGE", edge},
       weight: toWeight,
     });
     processContribution(src, {
-      contributor: {type: "NEIGHBOR", neighbor: {node: dst, edge}},
+      contributor: {type: "OUT_EDGE", edge},
       weight: froWeight,
     });
   }

src/core/attribution/graphToMarkovChain.test.js

@@ -109,44 +109,42 @@ describe("core/attribution/graphToMarkovChain", () => {
         .set(n1, [
           {contributor: {type: "SYNTHETIC_LOOP"}, weight: 1 / 13},
           {
-            contributor: {type: "NEIGHBOR", neighbor: {node: n2, edge: e1}},
+            contributor: {type: "OUT_EDGE", edge: e1},
             weight: 3 / 10,
           },
           {
-            contributor: {type: "NEIGHBOR", neighbor: {node: n3, edge: e3}},
+            contributor: {type: "OUT_EDGE", edge: e3},
             weight: 3 / 16,
           },
         ])
         .set(n2, [
           {contributor: {type: "SYNTHETIC_LOOP"}, weight: 1 / 10},
           {
-            contributor: {type: "NEIGHBOR", neighbor: {node: n1, edge: e1}},
+            contributor: {type: "IN_EDGE", edge: e1},
             weight: 6 / 13,
           },
           {
-            contributor: {type: "NEIGHBOR", neighbor: {node: n3, edge: e2}},
+            contributor: {type: "OUT_EDGE", edge: e2},
             weight: 3 / 16,
           },
         ])
         .set(n3, [
           {contributor: {type: "SYNTHETIC_LOOP"}, weight: 1 / 16},
           {
-            contributor: {type: "NEIGHBOR", neighbor: {node: n2, edge: e2}},
+            contributor: {type: "IN_EDGE", edge: e2},
             weight: 6 / 10,
           },
           {
-            contributor: {type: "NEIGHBOR", neighbor: {node: n1, edge: e3}},
+            contributor: {type: "IN_EDGE", edge: e3},
             weight: 6 / 13,
           },
           {
-            contributor: {type: "NEIGHBOR", neighbor: {node: n3, edge: e4}},
-            // this loop, as an out-edge
-            weight: 3 / 16,
+            contributor: {type: "IN_EDGE", edge: e4},
+            weight: 6 / 16,
           },
           {
-            contributor: {type: "NEIGHBOR", neighbor: {node: n3, edge: e4}},
-            // this loop, as an in-edge
-            weight: 6 / 16,
+            contributor: {type: "OUT_EDGE", edge: e4},
+            weight: 3 / 16,
           },
         ]);
       const canonicalize = (map) =>