Call gc.collect() multiple times in unit test.

In an upcoming change tf.Tensor is switching to proper Python GC. It need several calls to gc.collect() to break reference cycles.

PiperOrigin-RevId: 628422487

Author: rainwoodman

tensorflow_probability/python/layers/internal/distribution_tensor_coercible_test.py

@@ -305,10 +305,14 @@ def testTypeObjectLeakage(self):
     # These weakrefs could potentially come from tf.function.variables.
     dist = layer(x)
     gc.collect()
+    gc.collect()
+    gc.collect()
     before_objs = len(gc.get_objects())
     for _ in range(int(1e2)):
       dist = layer(x)
     gc.collect()
+    gc.collect()
+    gc.collect()
     after_objs = len(gc.get_objects())
     del dist