Add gradients for tf.batchNormalization4d

[caisq]

support gradients for x, mean, variance, offset and scale.

A step toward fulfilling: tensorflow/tfjs#20

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This change is 

[caisq]

cc @easadler

[dsmilkov]

Thank you! 2 small comments. Feel free to submit after.

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Reviewed 1 of 2 files at r1, 1 of 1 files at r2.
Review status: all files reviewed at latest revision, all discussions resolved, some commit checks failed.

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src/ops/batchnorm.ts, line 18 at r2 (raw file):

 */

import {rsqrt} from '..';

this was probably generated by the editor. Add explicitly the filename

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src/ops/batchnorm.ts, line 199 at r2 (raw file):

    const der = (dy: Tensor) => {
      util.assert(

move these asserts to the top of the batchNorm body (outside of the der() function)

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Comments from Reviewable

[dsmilkov]

Review status: all files reviewed at latest revision, 2 unresolved discussions, some commit checks failed.

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src/ops/batchnorm_test.ts, line 19 at r2 (raw file):

import * as tf from '../index';
import {onesLike, scalar} from '../index';

in tests, we've been usually calling tf.method() instead of method(). not feeling too strongly.

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Comments from Reviewable

[caisq]

Review status: all files reviewed at latest revision, 3 unresolved discussions, some commit checks failed.

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src/ops/batchnorm.ts, line 18 at r2 (raw file):

Previously, dsmilkov (Daniel Smilkov) wrote…

this was probably generated by the editor. Add explicitly the filename

Done.

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src/ops/batchnorm.ts, line 199 at r2 (raw file):

Previously, dsmilkov (Daniel Smilkov) wrote…

move these asserts to the top of the batchNorm body (outside of the der() function)

Done.

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src/ops/batchnorm_test.ts, line 19 at r2 (raw file):

Previously, dsmilkov (Daniel Smilkov) wrote…

in tests, we've been usually calling tf.method() instead of method(). not feeling too strongly.

Done.

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Comments from Reviewable