fix: fuse optimizer oom and make it stateless

[wangraying]

BREAKING CHANGE:

• contrib: bagua.torch_api.contrib.FusedOptimizer is now removed and replaced by bagua.torch_api.contrib.fuse_optimizer

[todo]

async algorithm conflict with fused optimizer, can only support flattened inplace bucket.

bagua/bagua/torch_api/algorithms/async_model_average.py

Lines 84 to 89 in 2fd9eb2

	# TODO: async algorithm conflict with fused optimizer, can only support flattened inplace bucket.
	assert do_flatten, "async does not support `do_flatten=False` at present."
	if self.step_id < self.warmup_steps:
	return super().tensors_to_buckets(tensors, do_flatten)
	all_tensors = []

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This comment was generated by todo based on a TODO comment in 2fd9eb2 in #207. cc @BaguaSys.

[todo]

qadam optimizer maintain `step_id` in its state

bagua/bagua/torch_api/algorithms/q_adam.py

Lines 47 to 50 in 2fd9eb2

	# TODO: qadam optimizer maintain `step_id` in its state
	self.step_id = 0
	self.warmup_steps = warmup_steps

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This comment was generated by todo based on a TODO comment in 2fd9eb2 in #207. cc @BaguaSys.