Migrate binary op to use logical sampling and add broadcasting support

[dsmilkov]

This change is 

[nsthorat]

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Reviewed 17 of 17 files at r1.
Review status: all files reviewed at latest revision, 2 unresolved discussions.

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src/math/math.ts, line 717 at r1 (raw file):

        `Error in scalarDividedByArray: first argument must be rank 0, but ` +
            `got NDArray of rank ${c.rank}.`);
    return this.divide(c, a) as T;

Can you update the graph methods, add, multiply, divide, sub to use the broadcast methods? You'll clean up a lot of code there

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src/math/webgl/shader_compiler.ts, line 312 at r1 (raw file):

      vec2 resTexRC = floor(gl_FragCoord.yx);
      float index = dot(resTexRC, vec2(${outTexShape[1]}.0, 1.0));
      index = mod(index, ${inSize}.0);

Tiny optimization: pass the broadcast bit in and splice this

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Comments from Reviewable

[dsmilkov]

Reviewed 10 of 17 files at r1.
Review status: 13 of 17 files reviewed at latest revision, 1 unresolved discussion, all commit checks successful.

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src/math/math.ts, line 717 at r1 (raw file):

Previously, nsthorat (Nikhil Thorat) wrote…

Can you update the graph methods, add, multiply, divide, sub to use the broadcast methods? You'll clean up a lot of code there

Per offline discussion, backprop is non-trivial for general broadcasting - better to do in another PR.

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src/math/webgl/shader_compiler.ts, line 312 at r1 (raw file):

Previously, nsthorat (Nikhil Thorat) wrote…

Tiny optimization: pass the broadcast bit in and splice this

Done.

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Comments from Reviewable

[dsmilkov]

This migrates more ops, tracked in #18