[bug] Fix potential bug of lang::Program that could be double finalized

[PGZXB]

Related issue = #4401, #5524

The bug is reflected in:

• https://github.com/taichi-dev/taichi/runs/7540400411?check_suite_focus=true DeviceAllocation, double deallocated
• https://github.com/taichi-dev/taichi/runs/7539283313?check_suite_focus=true Program::num_instances_ == -1, double finalized
• https://github.com/taichi-dev/taichi/runs/7538198742?check_suite_focus=true MemoryPool, double terminated

BUT I DON'T KNOW WHY AND WHEN Program::finalize WILL BE DOUBLE CALLED.

(Only when tests are run on multi-thread (and with offline_cache=True) is the bug triggered.

cc: @k-ye @strongoier @lin-hitonami

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[lin-hitonami]

The program::finalize may be called twice here.

taichi/python/taichi/lang/impl.py

Lines 370 to 375 in 8309129

	def clear(self):
	if self.prog:
	self.prog.finalize()
	self.prog = None
	self._signal_handler_registry = None
	self.materialized = False

[lin-hitonami]

LGTM!