change: NewVar() to Var()

[dzhwinter]

fix #4722
fix #4778

[helinwang]

LGTM++

[tonyyang-svail]

I think the issue is related to Scope::NewVar. BlockDescBind::NewVar should not be changed.

[dzhwinter]

It's the same with Scope, also do the same logic of find or create. Which we want to make it more clearly.

[Superjomn]

agree that NewVar is not resonable.
may be the implementation is wrong, an function called NewVar should just create a new var, raise error or return false if an variable exists.
We can add a new interface called GetOrCreateVar, and update the implementation of NewVar.

not just delete it.

FindVar matches NewVar grammerly
but not GetOrCreateVar

[helinwang]

@Superjom @dzhwinter From my understanding some of the current cpp implementations rely on NewVar returns a var if it's already created. Maybe we can create a new NewVar implementation that return false or raise exception when a var already exists. But change all current NewVar invocation to GetOrCreateVar, because currently noNewVar invocation is handling the error or exception and some of the NewVar invocation actually means to use GetOrCreateVar. And later the original author who wrote the NewVar invocation can change back to NewVar. Otherwise it will take @dzhwinter too much effort to figure out everyone's intension.

[Superjomn]

I think, most of the current usage of NewVar just use it to creates a new Variable semantically.
Better to change the NewVar's implementation to simply create new variable, not just replace all the usages using a more higher level interface called GetorCreateVar.
@helinwang

[dzhwinter]

We should only provide one interface to create a variable. Otherwise, the user's code may be ugly mixed with NewVar and GetOrCreateVar.
Throw exception is also not good enough as well. It's not a big deal that people want to reuse a delared variable, soGetOrCreateVar cover the use case of NewVar.

[Superjomn]

LGTM

[dzhwinter]

This PR depends #4728

[helinwang]

LGTM!

[helinwang]

LGTM

[jacquesqiao]

var should mean getOrCreate

[dzhwinter]

Done.

So many conflicts, I will merge it ASAP.