[TVMScript] Allow T.target("device", host="host") to specify host

[Lunderberg]

Prior to this commit, the TargetNode::host could be specified in TVMScript as part of the config dictionary, under the key "host". However, this required all other device parameters to be explicitly specified, rather than using any of the short-hand string representations. This commit forwards the host argument from TVMScript's T.target method to tvm.target.Target, allowing both the device and host to be specified using the shorthand string representation.

@T.prim_func
def before_this_commit():
    T.func_attr(
        {
            "target": T.target(
                {
                    "arch": "sm_86",
                    "host": {"keys": ["cpu"], "kind": "llvm", "tag": ""},
                    "keys": ["cuda", "gpu"],
                    "kind": "cuda",
                    "max_num_threads": 1024,
                    "tag": "",
                    "thread_warp_size": 32,
                }
            )
        }
    )
    T.evaluate(0)

@T.prim_func
def after_this_commit():
    T.func_attr({"target": T.target("cuda", host="llvm")})
    T.evaluate(0)

[tvm-bot]

Thanks for contributing to TVM! Please refer to the contributing guidelines https://tvm.apache.org/docs/contribute/ for useful information and tips. Please request code reviews from Reviewers by @-ing them in a comment.

• cc @Hzfengsy, @junrushao, @shingjan _{See #10317 for details}

_{Generated by tvm-bot}

[Lunderberg]

This PR is a subset of the functionality in #14862. As that PR has grown to a difficult-to-review size, I'm carving off independent chunks of it into separate PRs, after which #14862 can be rebased to a more manageable size.