Support string sorting

[tgsmith61591]

Describe the feature and the current behavior/state.

Currently string sorting in the graph is unsupported:

Does not work:

tf.sort(['d', 'a', 'b'])
InvalidArgumentError: Value for attr 'T' of string is not in the list of allowed values: bfloat16, half, float, double, int8, int16, int32, int64, complex64, complex128
	; NodeDef: {{node Neg}}; Op<name=Neg; signature=x:T -> y:T; attr=T:type,allowed=[DT_BFLOAT16, DT_HALF, DT_FLOAT, DT_DOUBLE, DT_INT8, DT_INT16, DT_INT32, DT_INT64, DT_COMPLEX64, DT_COMPLEX128]> [Op:Neg]

Works (but not in the graph):

>>> x = tf.constant(['d', 'a', 'b'])
>>> tf.convert_to_tensor(sorted(x.numpy()), tf.string)
<tf.Tensor: shape=(3,), dtype=string, numpy=array([b'a', b'b', b'd'], dtype=object)>

Can tf.sort be amended to allow string sorting?

Will this change the current api? How?

No

Who will benefit with this feature?

Anyone who might need to string-sort in the graph

Any Other info.

[ljwh]

I checked the code of operation, the problem is that tf.sort actually call the TOPK OP that does not support tf.string input type but only real number type.

REGISTER_OP("TopKV2")
    .Input("input: T")
    .Input("k: int32")
    .Output("values: T")
    .Output("indices: int32")
    .Attr("sorted: bool = true")
    .Attr("T: realnumbertype")  // real num type only
    .SetShapeFn(TopKShapeFn);

If change of string type is acceptable with interface design and the scope of change ?
if it is acceptable, I'd like to commit the c++ code of impl.

[ljwh]

@jvishnuvardhan

[preetham-salehundam]

I checked the code of operation, the problem is that tf.sort actually call the TOPK OP that does not support tf.string input type but only real number type.

REGISTER_OP("TopKV2")
    .Input("input: T")
    .Input("k: int32")
    .Output("values: T")
    .Output("indices: int32")
    .Attr("sorted: bool = true")
    .Attr("T: realnumbertype")  // real num type only
    .SetShapeFn(TopKShapeFn);

If change of string type is acceptable with interface design and the scope of change ? if it is acceptable, I'd like to commit the c++ code of impl.

May be have it included in tf.strings like

tf.strings.sort(['d', 'b', 'a'])
>> ['a', 'b', 'c']