[dattri.func.ihvp] Implement ihvp_at_x_ekfac

[sx-liu]

Description

1. Motivation and Context

This PR is the same as the closed one #79.

To complete the proposal in the issue #76.

2. Summary of the change

The implementation is basically the same as originally designed.

Specifically,

• Added ihvp_at_x_ekfac and its helper functions to dattri.func.ihvp.
• Added wrapper function manual and class MLPCache to dattri.func.ihvp.
• Slightly modified dattri.func.ihvp._check_input_size to support the EK-FAC method.

3. What tests have been added/updated for the change?

• [x] Unit test: Typically, this should be included if you implemented a new function/fixed a bug.

[sx-liu]

Seems like test_ihvp_datainf_nn is a bit shaky.

[jiaqima]

Seems like test_ihvp_datainf_nn is a bit shaky.

Could lower the threshold to 0.8.

[sx-liu]

Seems like test_ihvp_datainf_nn is a bit shaky.

Could lower the threshold to 0.8.

Should I do it here or in a separate PR?

[jiaqima]

Seems like test_ihvp_datainf_nn is a bit shaky.

Could lower the threshold to 0.8.

Should I do it here or in a separate PR?

You could just do it here. This shaky test is going to block other PRs if we merge this PR without lowering the threshold.