Revert "[Core] Improve Tensor serialisation (#18774)"

[njhill]

This reverts commit d73a945.

The changes broke various multimodal CI tests, e.g. https://buildkite.com/vllm/ci/builds/20971#019717d1-ac58-4aa3-b0b4-0bfa51e03fa2/213-7160.

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[njhill]

Moved to draft since we'll prefer to merge #18860 if it fixes the issue.

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @njhill.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork

[njhill]

Resolved by #18860