Megatron has left-over memory after refit completed

[yuki-97]

Ideally the memory should be exactly equal to 0 after refit completed, but megatron will have some left.

>       assert current_allocated == 0.0, "Memory should be 0 after refit completed"
E       AssertionError: Memory should be 0 after refit completed
E       assert 0.00244140625 == 0.0

Repro:
Set train_backend == megatron in the below unit test and run it.

RL/tests/unit/models/generation/test_vllm_generation.py

Lines 1667 to 1669 in ba46741

	# megatron still holds little memory after refit, so we only test dtensor now
	@pytest.mark.parametrize("train_backend", ["dtensor_v1", "dtensor_v2"])
	def test_vllm_weight_update_memory(cluster, tokenizer, train_backend):

.