Bump up shard_count for stochastic_cast_op_test from 10 to 20

PiperOrigin-RevId: 560753324

tensorflow/compiler/tests/BUILD

@@ -1596,7 +1596,7 @@ tf_xla_py_strict_test(
     # TODO(b/232442915): Enable MLIR.
     enable_mlir_bridge = False,
     python_version = "PY3",
-    shard_count = 10,
+    shard_count = 20,
     tags = [
         "no_pip",  # TODO(b/149738646): fix pip install so these tests run on kokoro pip
         "optonly",