[dmlc/tvm] [RFC][Relay] Port Relay passes to pass manager (#3146)

[Zhi <no...@github.com>]

# Porting Relay Passes to Pass Manager
As the pass manager framework has been merged, we should start to move passes to the pass manager. This RFC proposes the plans to move the Relay passes.

## Proposal (take constant folding as an example):
The proposal needs to solve problems from both the backend and the frontend. As this could be a lot of refactoring work, we will mainly focus on the API changes of the pass functions first, but leave others (like resolving dependency) as follow-up work.

### C++ Backend
The C++ backend implements the functionality of a certain pass as the `pass_func` for `module_pass` and `function_pass`, i.e. constant folding in this example. Some passes are passed with a `Module` to identify global vars and/or help report errors, but others are not.

1. Pass `Module` to every pass, or have a global `Module`? When `Module` is not passed, we can call `GetModule`, like `FromExpr`. For backward compatibility, we can default it to null. Therefore, users can still invoke the pass as before. All frontend converters should return a module as well.
2. Previously, many passes are not aware of PassContext. We probably should pass PassContext into a pass so that the internal context, e.g. ErrorReporter, can be used by each pass. 
3. How can a pass provide the frontend with the most convenient API so that it could be invoked easily?

```c++
class ConstantFolder : public ExprMutator {
 private:
  PassContext pass_ctx;
  // Other members are omitted..
};

Expr FoldConstant(const Expr& expr, const PassContext& ctx) {
  return ConstantFolder(CreateInterpreter(...), ctx).Mutate(expr);  
}  
  
TVM_REGISTER_API("relay._ir_pass.FoldConstant")  
.set_body([](TVMArgs args, TVMRetValue *ret) {
	PassContext ctx = args[1];
	if (!ctx.defined()) {
		ctx = PassContext();
	}  
    *ret = FoldConstant(args[0], ctx);  
});
```
### Python frontend

Python frontend performs the following tasks
- Link to a pass function, i.e. `ir_pass.fold_constant`
- Register the pass using flattened pass info, e.g. `opt_level`, `name`, and `required` passes.
- Setup `PassContext` when necessary. Otherwise, the backend will create it.
- Create a module for pass execution if it is not passed. 
- Have an API to invoke pass functions in different manner?
	- The old style invocation, `mypass(mod/expr)`, depending on the type of pass, e.g. `ir_pass.fold_constant(expr)`
	- Through pass manager 
```python
	# Create a module for all passes?
        mod = relay.Module({})
        
	# Method 1, Use decorator:
	@ir_pass.function_pass(opt_level=opt_level, name=pass_name)
	def mypass(expr, ctx):
		class Transform(ExprVisitor):
			def __init__():
			def __call__(expr, ctx):
		return Transform(expr, ctx)
	mypass(mod)
		
	# Method 2, pass registered passes directly:	
        pass_func = ir_pass.fold_constant
        cf = ir_pass.function_pass(pass_func, opt_level=2, name=”FoldConstant”, required=[])
 	cf(mod)	
```

## Next Steps
- One pass may call another pass directly. We should setup the `required` field and resolve dependencies. After that, this pass should be able to access the returned analysis/optimization results.
- How to represent a `Pass` using pass info, particularly pass name. To resolve pass dependency, we may only have the `required` passes. We then need to create theses pass and execute them.


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Re: [dmlc/tvm] [RFC][Relay] Port Relay passes to pass manager (#3146)

[Tianqi Chen <no...@github.com>]

consolidate to #3202 

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Re: [dmlc/tvm] [RFC][Relay] Port Relay passes to pass manager (#3146)

[Zhi <no...@github.com>]

For anyone who is interested in this, please comment. We appreciate your thoughts and suggestions. @MarisaKirisame and I will start working on it once we get some cycles. 

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Re: [dmlc/tvm] [RFC][Relay] Port Relay passes to pass manager (#3146)

[Tianqi Chen <no...@github.com>]

Closed #3146.

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Re: [dmlc/tvm] [RFC][Relay] Port Relay passes to pass manager (#3146)

[雾雨魔理沙 <no...@github.com>]

I had talked to @jroesch , and we think that there is two thing: the function (what we currently have), and the pass.
the pass is essentially a function from (Module, PassContext) to Module (yes, only Module), while the function can has different form - it may take an expr, or it may take a module, or even more info, etc.
the pass also should probably be done in C++, as we have the C++ build system merged recently.

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