[Bugfix][TIR][VTA] Update host-side target, even without device func

[Lunderberg]

This resolves an issue introduced by the combination of #14918 and
#14945. The bug occurred for targets that do not require device-side codegen, but do require a device_type other than kDLCPU. It wasn't caught by CI, as the issue only occurred with the combination of both PRs.

1. [TIR] SplitHostDevice, handle subroutines #14918 updated SplitHostDevice to only modify the "target" attribute when a device-side function has been extracted.

2. For VTA, there is no device-side function, as everything is done through host-side API calls.

3. From (1) and (2), the VTA examples kept the target T.target("ext_dev", host="llvm") after the SplitHostDevice pass, instead of being updated to T.target("llvm").

4. [TIR] Restrict tir.transform.CombineContextCall to host functions #14945 restricted CombineContextCall to only apply to host-side passes.

5. From (4) and (5), the CombineContextCall pass was no longer applied to the VTA context calls.

This PR fixes SplitHostDevice, updating the target from T.target("ext_dev", host="llvm") to T.target("llvm"), even if no device sections have been extracted from the function.

[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, @quic-sanirudh, @shingjan _{See #10317 for details}

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[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, @quic-sanirudh, @shingjan _{See #10317 for details}

_{Generated by tvm-bot}