[TARGET] Cleanup the target_host usage to new target style.

[tqchen]

As we move towards the new Target system, most of the target specifications in the compiler already moved to the new style. This PR aims to cleanup the rest in the codebase.

• Add deprecation warnings to functions with target_host parameters.
• Update the build usage to new target style.

[Mousius]

Hi @tqchen,

Is there an RFC or tracking issue for this you can link to this?

[junrushao]

@Mousius The RFC is here: https://discuss.tvm.apache.org/t/rfc-tvm-target-specification/6844

[junrushao]

CC: @zxybazh

[tqchen]

These were the left over items that should have been done as part of the Target system refactor. Most of the tutorials already moved to the new styles in previous refactor by @zxybazh , this PR aims to cleanup the rest.

[zxybazh]

Generally the PR looks good to me and thanks TQ for the very detailed follow up and adding warnings for target host deprecation. It's long been my hope that we can completely get rid of target host as an addtional argument and have a concise and united target representation. I am very supportive of the change and it takes a lot of effort to change all occurences of target host.

It's very easy to overlook some function calls to give warnings so here I propose to update the check_and_update_host_consist function as follows to check if we are dispatching a new target host for any target with no current target host.

    @staticmethod
    def check_and_update_host_consist(target, host=None, target_is_dict_key=True):
        """A helper function that merges a legacy "target, target_host" pair, then returns
        the merged target and its host field. The function is for legacy target and target
        host pair only, and should not be used in the new target system.

        Parameters
        ----------
        target : Union[str, Dict[str, Any], Target]
            The target or heterogeneous target
        host : Union[str, Dict[str, Any], Target, None]
            The target host
        target_is_dict_key : Bool
            When the type of target is dict, whether Target is the key (Otherwise the value)
        """
        if target is None:
            assert host is None, "Target host is not empty when target is empty."
            return target, host
        warning_flag = False
        if isinstance(target, dict) and "kind" not in target:
            new_target = {}
            for tgt, mod in target.items():
                if not target_is_dict_key:
                    tgt, mod = mod, tgt
                if isinstance(tgt, (dict, str, Target)):
                    tgt, host = Target.check_and_update_host_consist(tgt, host)
                if not target_is_dict_key:
                    tgt, mod = mod, tgt
                new_target[tgt] = mod
            target = new_target

        else:
            warning_flag = (host is not None) and (target.host is None)
            target = Target(target, host)
            host = target.host
        if warning_flag:
            warnings.warn(
                "target_host parameter is going to be deprecated. "
                "Please pass in tvm.target.Target(target, host=target_host) instead."
            )
        return target, host

[zxybazh]

Shall we also add the warning to extract_from_multiple_program as well?

[zxybazh]

This part seems to be obsolete after the change, would be great to remove.

[tqchen]

since we only issue an warning, we might still want to keep this one

[zxybazh]

The type is already checked when doing check_and_update_host_consist when it calls target = Target(target, host), see here. I'll remove it and check if there are other obsolete parts in my follow up PR.

[tqchen]

Thanks @zxybazh , given check_and_update_host_consist can be called multiple times, it may not be desirable to do so(for the second time it is called). So I think we can still take the explicit approach. The suggestion of extract_from_multiple_program is great, can you send a followup PR?

[zxybazh]

Sounds good. I'll send a follow up PR.