[core] Make matMul accept mismatching batch dimensions when one input has batch 1.

[annxingyuan]

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This change is 

[pyu10055]

Thanks!

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

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tfjs-core/src/ops/mat_mul_test.ts, line 79 at r1 (raw file):

    const c = tf.matMul(a, b);
    const data = await c.data();
    console.log(Array.from(data));

remove log

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tfjs-core/src/ops/mat_mul_test.ts, line 82 at r1 (raw file):

    // expect(c.shape).toEqual([1, 2, 2, 3, 1]);
  });
  // fit('unequal ranks', async () => {

should this test be enabled?

[annxingyuan]

Sorry guys - I somehow accidentally merged this... but I reverted it (70c823f).

Here's a new PR with the same code: #4100

Really sorry for the confusion - I think I had this window open for a while and then was trying to look at build logs but the window wasn't responding so I clicked a few times and then somehow merged... it's never happened before - I think I just need to refresh github windows that have been open for a while from now on...