Add GetBasePtr interface in paddle::memory

[From00]

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Describe

After PR #38853 merged, the memory::allocation::Allocation is a derived class of pten::Allocation, and the Alloc interface in malloc.h return a pten::Allocation type. Since the base_ptr function introduced in #37978 is implenmemted in memoy::allocation::Allocation, now we have to get the base-pointer from an allocation like:

std::shared_ptr<pten::Allocation> allocation = memory::AllocShared(place_, size);
void* base_ptr = static_cast<memory::allocation::Allocation*>(allocation.get())->base_ptr();

It is so inconvenient compared with the previous method:

void* base_ptr = allocation->base_ptr();

This PR adds a GetBasePtr interface in paddle::memory that hides the inelegant static_cast in AllocatorFacadePrivate, therefore we can get the base-pointer by a simple way like:

void* base_ptr = memory::GetBasePtr(allocation);

[zhiqiu]

LGTM