[REFACTOR][TE][TIR] Call::Halide => ProducerLoad, DSL/TIR decouple.

[tqchen]

In the HalideIR's design, DSL components and IR are mixed together.
For example, Call::Halide can containa reference to a function
constructed in the tensor expression language(DSL) stage.

While this coupled design simplifies certain aspect of the DSL construction,
it prevents the low-level IR to evolve as a clean standalone IR:

• The additional tensor expression provided in the function is opaque to the IR
  and may become obsolete as we transform them.
• The duplication of the information in the DSL tensor and IR makes it hard to
  design a stand-alone text format (when there are elements shared in the tensor
  expression and normal statements).

This PR aims to clearly de-couple the TIR from high-level DSL structures(tensor expression),
while still provide clear extensions to build DSLs on top of the TIR.

We introduce a DataProducer as a base class for high level tensor expressions objects
that produce data. We then introduce ProducerLoad to replace the Call::Halide usage,
so that the Call node can always be self contained and used for low-level calls.

The high-level tensor expression DSL can still generate a PrimExpr that contains a ProducerLoad.
These PrimExprs contains fragments of information that can be combined together to
generate a low-level TIR PrimFunc.

We also state clearly that DataProducer should not appear in any TIR PrimFunc.
Instead, the high-level DSL layer should lowered DataProducers to Buffers and TIR statements
that produces these buffers. We can further provide verifications to validate such invariance.

Changes:

• Introduce DataProducer to serve as a base class for Tensor in tensor expressions.
• Migrate use of Call::Halide to ProducerLoad
• Migrate the other usages of Calls.

We will also create follow-up PRs to migrate the remaining two DSL related IR nodes(Realize/Provide)
to use the DataProducer.

[tqchen]

cc @merrymercy @junrushao1994 @Hzfengsy @yzhliu @icemelon9 @ZihengJiang @were @zhiics