[minor] fix doc and assert and test around percent (#1067)

fairscale/experimental/wgit/signal_sparsity.py

@@ -435,11 +435,11 @@ def random_sparse_mask(dense: Tensor, percent: float, dim: int) -> Tensor:
         dense (Tensor):
             Input dense tensor (no zeros).
         percent (float):
-            Percent of non-zeros.
+            Percent of non-zeros (0, 100].
         dim (int):
             Dimension on which the random sparse mask is computed.
     """
-    assert percent > 0, percent
+    assert percent > 0 and percent <= 100, percent
     rand = torch.rand_like(dense)
     ones = torch.ones_like(dense)
     k = _get_k_for_topk(percent, None, dense.shape[dim])

tests/experimental/wgit/test_signal_sparsity.py

@@ -436,10 +436,10 @@ def test_random_sparse_mask(device):
         pytest.skip("no GPU")
 
     dense = torch.tensor([0.5000, 0.6000, 0.7000, 0.8000, 0.9000]).to(device)
-    mask = random_sparse_mask(dense, 0.2, 0)
+    mask = random_sparse_mask(dense, 20, 0)
     assert mask.sum() == 1
     for d in [0, 1]:
         dense = torch.rand(100, 100).to(device)
-        mask = random_sparse_mask(dense, 0.01, d)
+        mask = random_sparse_mask(dense, 1, d)
         assert objects_are_equal(mask.sum(dim=d), torch.ones(100).to(device), raise_exception=True)
         assert mask.sum() == 100