The stats initialization in the AbstractMemory allocates the space
according to the max requestors of the System. This may cause issues in
multiple system simulation.
Given there are two system A and B. A has one requestor and a memory,
while B has two requestors. When the requestor with requestor id 2
sending requests to the meomry in A, the simulator would crash because
requestor id 2 is out of the allocated space.
Current solution is adding a SysBridge between across A and B which
would rewrite the requestor id to a valid one. This solution works but
it needs to the bridge at the correct boundary which may not easy. In
addition, the stats would record a mapped data which may not accurate.
To reduce the complexity, we add an flag to AbstractMemory to control
the stats. If users don't want the statistics and want to solve the
cross system issue simply, users can disable the statistics collection.
We also makes the flag by default True to not disturb current users.
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