[TensorExpr] Add a wrapper for all expr and stmt pointers. (#63195)

[TensorExpr] Add a wrapper for all expr and stmt pointers. (#63195)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/63195

This helps us to later switch from using KernelArena with raw pointers
to shared pointers without having to change all our source files at
once.

The changes are mechanical and should not affect any functionality.

With this PR, we're changing the following:
 * `Add*` --> `AddPtr`
 * `new Add(...)` --> `alloc<Add>(...)`
 * `dynamic_cast<Add*>` --> `to<Add>`
 * `static_cast<Add*>` --> `static_to<Add>`

Due to some complications with args forwarding, some places became more
verbose, e.g.:
 * `new Block({})` --> `new Block(std::vector<ExprPtr>())`

Test Plan: Imported from OSS

Reviewed By: navahgar

Differential Revision: D30292779

Pulled By: ZolotukhinM

fbshipit-source-id: 150301c7d2df56b608b035827b6a9a87f5e2d9e9