[Frontend][Relay][Keras] Fix concatenate convert function in axis parsing

[jikechao]

This pr fixed the bug in #15174 and added some specific test cases for Concatenate operator.

cc @echuraev @Hzfengsy

[tvm-bot]

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

_{Generated by tvm-bot}

[jikechao]

The newly added test cases cannot pass the CI test. However, It can run well in my local environment. So wired it is.

[Hzfengsy]

Is it related to the Karas version?

[jikechao]

@Hzfengsy @junrushao Thanks for your comments and review.
My patch was incomplete and It only considered the layout as 'NHWC'. The CI failed when layout = 'NCHW' for the input will be transposed. I have corrected the patches. Could you help me review it again? Thanks again!

[echuraev]

LGTM! Thanks.

[jikechao]

@echuraev Thanks for your review.

[jikechao]

@tvm-bot rerun