Unused argument max_block_size in split_dense_variable()

[emailweixu]

max_block_size is not used in this function:

Paddle/python/paddle/fluid/distribute_transpiler.py

Lines 91 to 95 in f53beed

	def split_dense_variable(var_list,
	pserver_count,
	min_block_size=1024,
	max_block_size=1048576):

[typhoonzero]

This may be related to #9675. We can reduce the copy of split op and limit the max size of each parameter/gradient block, and send those blocks using an "IOthreadpool" ( which should contain much more threads than machine CPU cores).

And, in earlier tests result #7944, we see that split copying is not the major time-consuming operator, so we delayed this improvement.