Avoid std::copy_n in CopyKernel and IndexKernel

[cyyever]

This PR simplifies std::copy_n calls in CopyKernel and IndexKernel. std::copy_n is used to create a data pointer array from the input data pointers. However, more careful review reveals that the dest pointers are actually aliases of the original pointers. So we can removes the pointer manipulations.

cc @jgong5 @mingfeima @XiaobingSuper @sanchitintel @ashokei @jingxu10 @jerryzh168 @aditew01

[pytorch-bot]

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[cyyever]

@pytorchbot label "topic: not user facing"

[cyyever]

@pytorchbot rebase

[pytorchmergebot]

@pytorchbot started a rebase job onto refs/remotes/origin/viable/strict. Check the current status here

[pytorchmergebot]

Successfully rebased const_cast2 onto refs/remotes/origin/viable/strict, please pull locally before adding more changes (for example, via git checkout const_cast2 && git pull --rebase)

[cyyever]

@pytorchbot rebase -b main

[pytorchmergebot]

@pytorchbot started a rebase job onto refs/remotes/origin/main. Check the current status here

[pytorchmergebot]

Successfully rebased const_cast2 onto refs/remotes/origin/main, please pull locally before adding more changes (for example, via git checkout const_cast2 && git pull --rebase)

[albanD]

I'm not familiar with this code. But I'm surprised these would just be dead code we can just remove?
Would you be able to give some details for each change on what they're doing and why it's ok to do them?

[cyyever]

@albanD Yes, they are redundant code, I have added some more descriptions.

[Skylion007]

Is the test failure real?

[cyyever]

@pytorchbot rebase

[pytorchmergebot]

@pytorchbot started a rebase job onto refs/remotes/origin/viable/strict. Check the current status here

[pytorchmergebot]

Successfully rebased const_cast2 onto refs/remotes/origin/viable/strict, please pull locally before adding more changes (for example, via git checkout const_cast2 && git pull --rebase)

[cyyever]

Is the test failure real?

No, it was flaky.

[albanD]

Very nice simplification, thanks!

[albanD]

@pytorchbot merge

[pytorchmergebot]

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