Fix CPU fusedConv2d (activation) for NCHW format

[Linchenn]

ISSUE #6368
ISSUE #6354

For NCHW format, if fusedConv2d has a 1-D PReLU activation weights, CPU-backend does not apply it to the channel dimension, while currently just applying it to the last dimension of the input.

This PR does:

1. For CPU-backend, properly reshape PReLU activation weights to align with the NCHW format to compute fusedConv2d
2. Moves the shape check of PReLU activation weights from WebGL-backend to tfjs-core, because this checking is general for all backends.

I have manually tested the PReLU activation logics in this PR, using the tests from the PR #6373. These tests will be added when the PR #6373 is merged.

@lina128 , FYI, this is the twin PR with the PR #6379.

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This change is 

[pyu10055]

thank you for fixing the cpu issue, can the webgl NCHW tests be moved to core within this PR?

Reviewable status: 0 of 1 approvals obtained (waiting on @Linchenn)

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tfjs-backend-cpu/src/kernels/FusedConv2D.ts line 87 at r1 (raw file):

          backend, result, activation, reshapedAlpha, leakyreluAlpha);
      backend.disposeIntermediateTensorInfo(reshapedAlpha);
      console.log('Tested');

remove the log here.

[Linchenn]

Before moving the tests to tfjs-core, we have to enable tfjs-core's fusedConv2d-NCHW and ban some backend's fusedConv2d-NCHW. This involves many codes, so we could do it separately in another PR (#6373 is in progress).

Reviewable status: 0 of 1 approvals obtained (waiting on @mattsoulanille and @pyu10055)

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tfjs-backend-cpu/src/kernels/FusedConv2D.ts line 87 at r1 (raw file):

Previously, pyu10055 (Ping Yu) wrote…

remove the log here.

Done.

[mattsoulanille]

LGTM, and moving the tests in a separate PR sounds good.

Reviewed 2 of 3 files at r1, 1 of 1 files at r2, all commit messages.
Reviewable status: complete! 1 of 1 approvals obtained (waiting on @pyu10055)

[pyu10055]

Reviewed all commit messages.
Reviewable status: complete! 2 of 1 approvals obtained (waiting on @pyu10055)

[pyu10055]

Reviewable status: complete! 2 of 1 approvals obtained