[refactor] Clean up keep_raw

[squarefk]

Few places use keep_raw from the python Matrix class constructor. Currently, the parameter list is too long. It should be great to remove keep_raw option and find an alternative way to do so.

taichi/python/taichi/lang/ndrange.py

Lines 45 to 47 in 4d55957

	def __iter__(self):
	for ind in self.r:
	yield ti.Vector(list(ind), keep_raw=True)