Add destructor for XLAGraphExecutor to free resource

[Zantares]

This PR is to add a destructor for XLAGraphExecutor, to trigger the destructor of PJRT class Executable/LoadedExecutable. This helps to do some special works (e.g. clear disk) when the PJRT program is over.

Backgroud

The PJRT plugins may want to do some special operations when the Executable/LoadedExecutable is destructed, but in Torch-XLA, the Executable/LoadedExecutable will never be destructed since a raw pointer computation_cache_ has stored these objects and is never released:

• The raw pointer computation_cache_ is created in CreateComputationCache()
• The PJRT Executable/LoadedExecutable is added to computation_cache_ in https://github.com/pytorch/xla/blob/master/torch_xla/csrc/xla_graph_executor.cpp#L1595
• The foo class XLAGraphExecutor is used as a global static instance and will never clear or delete this pointer

Detail

Add a destructor for XLAGraphExecutor, to clear and delete this raw pointer.

[qihqi]

thanks!