IRMutator and IRVisitor were the main data structures for doing low level IR visiting.
As the project evolves, we start to introduce more powerful variants such as StmtFunctor and ExprFunctor.
This PR brings new classes that allows us to migrate the visitor mutator to be sub-class of these functors.

List of changes:

- Create separate class for ExprMutator and StmtMutator, following convention used in relay.
- Introduce copy-on-write to StmtMutator that can later benefit the statement mutations
  if we use move semantics and keep a single copy of stmt.
- Move two generic visit mutate util to use the new classes.

We will send followup PRs to migrate the existing passes that use the legacy visitors
to the new one.

* [FRONTEND][TF] Add conv3d

* fix high rtol

* [REFACTOR][OBJECT] Consoldiate NodePtr/Ref/Hash/Equal and macros to Object.

Historically, we have classes like NodePtr/Ref/HashEqual.
After unified object protocol, these names are just alias of the object counterpart.
Moreover, there are helper macros defined over the places for defining these object.

This PR consoldiate the terminologies into the corresponding ones
in the Object system so we have a clean and consistent API moving forward.

* Update include/tvm/attrs.h

Co-Authored-By: Wei Chen <ipondering.weic@gmail.com>

* fix compilation

Co-authored-by: Wei Chen <ipondering.weic@gmail.com>

* Refactor to use IsOp utility

* retrigger CI

* [REFACTOR][RUNTIME] Move NDArray to Object System.

Previously NDArray has its own object reference counting mechanism.
This PR migrates NDArray to the unified object protocol.

The calling convention of NDArray remained intact.
That means NDArray still has its own type_code and
its handle is still DLTensor compatible.

In order to do so, this PR added a few minimum runtime type
detection in TVMArgValue and RetValue only when the corresponding
type is a base type(ObjectRef) that could also refer to NDArray.

This means that even if we return a base reference object ObjectRef
which refers to the NDArray. The type_code will still be translated
correctly as kNDArrayContainer.
If we assign a non-base type(say Expr) that we know is not compatible
with NDArray during compile time, no runtime type detection will be performed.

This PR also adopts the object protocol for NDArray sub-classing and
removed the legacy NDArray subclass protocol.
Examples in apps/extension are now updated to reflect that.

Making NDArray as an Object brings all the benefits of the object system.
For example, we can now use the Array container to store NDArrays.

* Address review comments

* cublaslt added

* fix lint

* address comments

* address more comments

* Trigger CI

* Trigger CI

* [GraphRuntime] Support parameter out in the graph runtime debug

* Dummy commit to trigger build

* [FRONTEND][TF] conv3d_transpose 'SAME' support kernel more than 1x1

* revised per as review comments

* add more fallback wolkaround to make all tests pass

* [TOPI] add 3D upsampling Op.

* fix lint issues

* change align_corners to coordinate_transformation_mode

* fix resize3d half_pixel

* make a simple function and clean up trilinear_resize3d_python

* fix doc

* [AutoTVM][TOPI] NHWC conv2d templates (spatial pack) for ARM

As some frontends (tflite for example) are using NHWC as the default
layout, we are enabling NHWC schedule templates in TOPI and AutoTVM.

* some comments fix

* [NEWS] add v0.6 release

* remove link prefix

* fix issue number

* fix python exception creation in Windows

* better string conversion for msvc

* fix cpp style issue

Before the unified object protocol, we support pass
additional extension objects around by declaring a type as an extension type.
The old extension mechanism requires the types to register their
constructor and deleter to a VTable and does not enjoy the benefit of the
self-contained deletion property of the new Object system.

This PR upgrades the extension example to make use of the new object system
and removed the old Extension VTable.

Note that the register_extension funtion in the python side continues to work
when the passed argument does not require explicit container copy/deletion,
which covers the current usecases of the extension mechanism.

This PR cleans up the left over code for legacy verilog support which was experimental.
The new hardware backend path is now support by VTA via TSIM.

* Added tvm function stencil for subpixel operations to topi.

* Topi subpixel operators added and tested.

* Added subpixel attrs.

* Added depth_to_space relay attributes.

* depth_to_space fully working.

* Fixed NHWC shape bug.

* SpaceToDepth in and all tests passing.

* lint fixes.

* Added string include

* Fixed topi formatting.

* Added DCR/CDR mode to depthtospace operator.

* Remove NNVM compiler

* [VTA][Chisel] End-to-end Inference with Chisel VTA

* Update TensorAlu.scala

* [Relay] add max_pool3d in relay and TF converter

* fix comments

* [TEST] Remove nnvm related code in topi and test script

* Remove docs dep

dtype.h -> runtime/data_type.h

Changes:
- Rename all old reference of tvm::Type to DataType
- ExprNode.type -> ExprNode.dtype
- Expr.type() -> Expr.dtype()
- Change Expr related functions to expr_operator.
  - DataType::min() -> min_value(DataType)
  - DataType::max() -> max_value(DataType)
- Move type constructor Int, UInt, Float, Handle, Bool into DataType.
  - Int(bits) -> DataType::Int(bits)
  - UInt(bits) -> DataType::UInt(bits)