Use rootMemoryPool to construct execContext (#480)

Summary:
Pull Request resolved: #480

It originally use `memory::getDefaultScopedMemoryPool()` since `ExecCtx`
 constructor expects `unique_ptr<MemoryPool>`.

Since facebookincubator/velox@15eeddb,
`ExecCtx` now expects `MemoryPool*`, so we can use
`&velox::memory::getProcessDefaultMemoryManager().getRoot()` returned by
`TorchArrowGlobalStatic::rootMemoryPool`.

This makes TorchArrow always use this process default root memory pool.

Reviewed By: dracifer, vancexu

Differential Revision: D39010989

fbshipit-source-id: 3a383b01163e9866500a6fc8b091080b70684c6e

Author: wenleix

csrc/velox/column.cpp

@@ -553,9 +553,9 @@ velox::core::QueryCtx& TorchArrowGlobalStatic::queryContext() {
 }
 
 velox::core::ExecCtx& TorchArrowGlobalStatic::execContext() {
-  static auto pool = velox::memory::getDefaultScopedMemoryPool();
   static velox::core::ExecCtx execContext(
-      pool.get(), &TorchArrowGlobalStatic::queryContext());
+      TorchArrowGlobalStatic::rootMemoryPool(),
+      &TorchArrowGlobalStatic::queryContext());
   return execContext;
 }