StringConcat Operator

[adityagoel4512]

Description

This PR introduces a StringConcat operator, as raised in #5339.

StringConcat takes two string tensors as input and returns the elementwise concatenation of the strings in each tensor, with support for numpy-like broadcasting. The first input prefixes onto the second input in the output.

Examples are as follows:

StringConcat(["abc", "def"], [".com", ".net"])  ==> ["abc.com", "def.net"]
StringConcat(["tiger", "lion", "zebra"], ["s"]) ==> ["tigers", "lions", "zebras"]

This directly implements numpy.char.add and can be used to implement tf.strings.join. The following script can be used to validate this:

import numpy as np
from functools import reduce
import tensorflow as tf

onnx_concat_op = np.char.add

def tf_concat_op(inputs, separator=None):
    reduce_op = onnx_concat_op if separator is None else lambda x, y: onnx_concat_op(x, onnx_concat_o
p(separator, y))
    return reduce(reduce_op, inputs)

if __name__ == "__main__":
    data = ['abc', 'def']
    res1 = np.array(tf.strings.join(data).numpy()).astype(np.str_)
    res2 = tf_concat_op(data)
    np.testing.assert_equal(res1, res2)

    data = [['abc','123'],['def','456'],['ghi','789']]
    res1 = tf.strings.join(data).numpy().astype(np.str_)
    res2 = tf_concat_op(data)
    np.testing.assert_equal(res1, res2)

    data = [['abc','123'],['def','456']]
    res1 = tf.strings.join(data, separator=" ").numpy().astype(np.str_)
    res2 = tf_concat_op(data, separator=" ")
    np.testing.assert_equal(res1, res2)

Motivation and Context

Closes #5339.

[xadupre]

I don't think we should update test cases for acosh and other operator not related to StringConcat.

[gramalingam]

Looks good to me.

One minor thought/question: the op looks out of place in the nn folder/file. I wonder if we should create a new folder/file for string or text ? Other suggestions/inputs welcome.

[adityagoel4512]

Looks good to me.

One minor thought/question: the op looks out of place in the nn folder/file. I wonder if we should create a new folder/file for string or text ? Other suggestions/inputs welcome.

I'd be favour of a text directory.

[gramalingam]

LGTM, thanks!

[cbourjau]

I would be great to get this merged since several other PRs are also touching the text folder