support fused matmul

[pyu10055]

Support fusing MatMul + BiasAdd + Activation => _FusedMatMul

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This change is 

[dsmilkov]

Awesome! 2 small comments.

Reviewed 9 of 9 files at r1.
Reviewable status: complete! 1 of 1 approvals obtained (waiting on @dsmilkov and @pyu10055)

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tfjs-converter/python/tensorflowjs/converters/fuse_prelu.py, line 160 at r1 (raw file):

    updated_alpha.append(neg_alpha_op.name)

def fuse_prelu_with_fused_conv2d(input_graph_def):

rename to fused_prelu_with_conv2d_or_matmul

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tfjs-converter/python/tensorflowjs/converters/graph_rewrite_util.py, line 23 at r1 (raw file):

# Custom op name for fused depthwise conv2d
FUSED_DEPTHWISE_CONV2D = 'FusedDepthwiseConv2dNative'
FUSED_MATMUL = '_FusedMatMul'

add a comment on top saying that this starts with underscore because the name was created by grappler

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tfjs-converter/src/operations/executors/matrices_executor_test.ts, line 58 at r1 (raw file):

    });
    describe('_FusedMatMul', () => {
      it('should call tfc.fused.matMul', () => {

add a test with prelu as well