[LTC] Make some LazyGraphExecutor private data structures protected
Summary:
This pull request makes some LazyGraphExecutor private data structures protected such that XLAGraphExecutor can reuse them.
Here is the list:
1. DeviceLocker.
2. DeviceLockerArena.
3. DataCacheArena.
In addition, it also introduces LazyGraphExecutor::ResetTrimCounter() such that XLAGraphExecutor can reuse the trim counter.
Test Plan:
CI.
