bug: GraphKernel does not conform to AbstractKernel interface

[st--]

Bug Report

GPJax version:
v0.6

Current behavior:
GraphKernel.kernel expects full matrix, not individual data points.

Expected behavior:
GraphKernel.kernel should take individual data points.

Steps to reproduce:
Compare type annotations to other kernels (see #230)

[github-actions]

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Please update to the latest version of GPJax and check if that resolves the issue. Let us know if that works for you by leaving a comment.
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[github-actions]

There has been no activity on this PR for some time. Therefore, we will be automatically closing the PR if no new activity occurs within the next seven days.
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