[Bugfix] Fixes the redundancy parameter being used wrong in global negative sampling (#3657)

* oops

* test

src/array/cpu/negative_sampling.cc

@@ -27,7 +27,7 @@ std::pair<IdArray, IdArray> CSRGlobalUniformNegativeSampling(
     double redundancy) {
   const int64_t num_row = csr.num_rows;
   const int64_t num_col = csr.num_cols;
-  const int64_t num_actual_samples = static_cast<int64_t>(num_samples * redundancy);
+  const int64_t num_actual_samples = static_cast<int64_t>(num_samples * (1 + redundancy));
   IdArray row = Full<IdType>(-1, num_actual_samples, csr.indptr->ctx);
   IdArray col = Full<IdType>(-1, num_actual_samples, csr.indptr->ctx);
   IdType* row_data = row.Ptr<IdType>();

src/array/cuda/negative_sampling.cu

@@ -140,7 +140,7 @@ std::pair<IdArray, IdArray> CSRGlobalUniformNegativeSampling(
   auto dtype = csr.indptr->dtype;
   const int64_t num_row = csr.num_rows;
   const int64_t num_col = csr.num_cols;
-  const int64_t num_actual_samples = static_cast<int64_t>(num_samples * redundancy);
+  const int64_t num_actual_samples = static_cast<int64_t>(num_samples * (1 + redundancy));
   IdArray row = Full<IdType>(-1, num_actual_samples, ctx);
   IdArray col = Full<IdType>(-1, num_actual_samples, ctx);
   IdArray out_row = IdArray::Empty({num_actual_samples}, dtype, ctx);

tests/compute/test_sampling.py

@@ -892,10 +892,10 @@ def test_sample_neighbors_exclude_edges_homoG(dtype):
 
 @pytest.mark.parametrize('dtype', ['int32', 'int64'])
 def test_global_uniform_negative_sampling(dtype):
-    g = dgl.graph((np.random.randint(0, 20, (10,)), np.random.randint(0, 20, (10,)))).to(F.ctx())
-    src, dst = dgl.sampling.global_uniform_negative_sampling(g, 20, False, True)
-    assert len(src) > 0
-    assert len(dst) > 0
+    g = dgl.graph(([], []), num_nodes=1000).to(F.ctx())
+    src, dst = dgl.sampling.global_uniform_negative_sampling(g, 2000, False, True)
+    assert len(src) == 2000
+    assert len(dst) == 2000
 
     g = dgl.graph((np.random.randint(0, 20, (300,)), np.random.randint(0, 20, (300,)))).to(F.ctx())
     src, dst = dgl.sampling.global_uniform_negative_sampling(g, 20, False, True)