[Static Runtime] Move MemoryPlanner out into memory_planner.cpp (#65123)

[Static Runtime] Move MemoryPlanner out into memory_planner.cpp (#65123)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/65123

This change re-reverts D30883290 (https://github.com/pytorch/pytorch/commit/0e11454d19e106ba6d5819c1147ca540cbce2943). D30883290 (https://github.com/pytorch/pytorch/commit/0e11454d19e106ba6d5819c1147ca540cbce2943) broke the OSS build since the change in this change implicitly removed the default move constructor of `StaticRuntime`.

```
ep 15 15:39:57 /var/lib/jenkins/workspace/benchmarks/static_runtime/deep_wide_pt_bench.cc:95:10: error: call to implicitly-deleted copy constructor of 'torch::jit::StaticRuntime'
Sep 15 15:39:57   return torch::jit::StaticRuntime(*smod);
Sep 15 15:39:57          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Sep 15 15:39:57 /var/lib/jenkins/workspace/torch/csrc/jit/runtime/static/impl.h:321:34: note: copy constructor of 'StaticRuntime' is implicitly deleted because field 'planner_' has a deleted copy constructor
Sep 15 15:39:57   std::unique_ptr<MemoryPlanner> planner_;
Sep 15 15:39:57                                  ^
Sep 15 15:39:57 /usr/bin/../lib/gcc/x86_64-linux-gnu/5.4.0/../../../../include/c++/5.4.0/bits/unique_ptr.h:356:7: note: 'unique_ptr' has been explicitly marked deleted here
Sep 15 15:39:57       unique_ptr(const unique_ptr&) = delete;
Sep 15 15:39:57       ^
Sep 15 15:39:57 /var/lib/jenkins/workspace/benchmarks/static_runtime/deep_wide_pt_bench.cc:99:9: error: call to implicitly-deleted copy constructor of 'torch::jit::StaticRuntime'
Sep 15 15:39:57    auto sr = getStaticRuntime();
Sep 15 15:39:57         ^    ~~~~~~~~~~~~~~~~~~
Sep 15 15:39:57 /var/lib/jenkins/workspace/torch/csrc/jit/runtime/static/impl.h:321:34: note: copy constructor of 'StaticRuntime' is implicitly deleted because field 'planner_' has a deleted copy constructor
Sep 15 15:39:57   std::unique_ptr<MemoryPlanner> planner_;
Sep 15 15:39:57                                  ^
Sep 15 15:39:57 /usr/bin/../lib/gcc/x86_64-linux-gnu/5.4.0/../../../../include/c++/5.4.0/bits/unique_ptr.h:356:7: note: 'unique_ptr' has been explicitly marked deleted here
Sep 15 15:39:57       unique_ptr(const unique_ptr&) = delete;
Sep 15 15:39:57       ^
Sep 15 15:39:57 2 errors generated.
```

This change fixes the issue by explicitly defining the default move constructor (courtesy of mikeiovine).

Original Summary:

This change moves `MemoryPlanner` out of impl.cpp into memory_planner.cpp.

`MemoryPlanner` performs an independent sub-task of static analysis of a graph, and creating memory planning, and allocating/deallocating managed Tensors.

This change will reduce merge conflicts as I work on MemoryPlanner more actively for output Tensor support.

Test Plan: - Confirm that OSS build went well (See External Tests section).

Reviewed By: mikeiovine

Differential Revision: D30983292

fbshipit-source-id: a59f407fa1123527824157268111144a1bf58116