[Sampling] handle fanout=-1 differently from fanout>0 in sample_etype_neighbors() (#4716)

src/array/cpu/rowwise_pick.h

@@ -284,7 +284,8 @@ COOMatrix CSRRowWisePerEtypePick(CSRMatrix mat, IdArray rows, IdArray etypes,
             // 1 end of the current etype
             // 2 end of the row
             // random pick for current etype
-            if (et_len <= num_picks[cur_et] && !replace) {
+            if ((num_picks[cur_et] == -1) ||
+                (et_len <= num_picks[cur_et] && !replace)) {
               // fast path, select all
               for (int64_t k = 0; k < et_len; ++k) {
                 rows.push_back(rid);

tests/compute/test_sampling.py

@@ -882,7 +882,7 @@ def test_sample_neighbors_etype_sorted_homogeneous(format_, direction):
     h_g = dgl.to_homogeneous(g)
     seed_ntype = g.get_ntype_id("u")
     seeds = F.nonzero_1d(h_g.ndata[dgl.NTYPE] == seed_ntype)
-    fanouts = F.tensor([6, 5, 4, 3, 2], dtype=F.int64)
+    fanouts = F.tensor([6, 5, -1, 3, 2], dtype=F.int64)
     h_g = h_g.formats(format_)
     if (direction, format_) in [('in', 'csr'), ('out', 'csc')]:
         h_g = h_g.formats(['csc', 'csr', 'coo'])