## What changes were proposed in this pull request?
`ctx.currentVars` means the input variables for the current operator, which is already decided in `CodegenSupport`, we can set it there instead of `doConsume`.
also add more comments to help people understand the codegen framework.
After this PR, we now have a principle about setting `ctx.currentVars` and `ctx.INPUT_ROW`:
1. for non-whole-stage-codegen path, never set them. (permit some special cases like generating ordering)
2. for whole-stage-codegen `produce` path, mostly we don't need to set them, but blocking operators may need to set them for expressions that produce data from data source, sort buffer, aggregate buffer, etc.
3. for whole-stage-codegen `consume` path, mostly we don't need to set them because `currentVars` is automatically set to child input variables and `INPUT_ROW` is mostly not used. A few plans need to tweak them as they may have different inputs, or they use the input row.
## How was this patch tested?
existing tests.
Author: Wenchen Fan <wenchen@databricks.com>
Closes#19803 from cloud-fan/codegen.
70221903f5