Add a symbolize transformer

[babacry]

Symbolizes single qubit gates by tags.

Example:
Input:

        0: ───X[phxz_0]───@───Y[phxz_1]───X───
                          │
        1: ───────────────@───────────────────

$ cirq.symbolize_single_qubit_gates_by_indexed_tags(c, tag_prefix="phxz")
Output:

        0: ───PhXZ(a=a0,x=x0,z=z0)───@───PhXZ(a=a1,x=x1,z=z1)───X───
                                     │
        1: ──────────────────────────@──────────────────────────────

where, a0,x0,z0,a1,x1,z1 are symbols.

Context: to be used in #6994.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.67%. Comparing base (716549e) to head (e2458d0).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7307   +/-   ##
=======================================
  Coverage   98.67%   98.67%           
=======================================
  Files        1111     1113    +2     
  Lines       97012    97057   +45     
=======================================
+ Hits        95728    95773   +45     
  Misses       1284     1284           

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[babacry]

@NoureldinYosri , phxz symbolizer transformer! Could you help review?

[NoureldinYosri]

overall looks good but lets make the tag a class instead of a string, for example

@attrs.frozen
class SymbolizeTag:
    prefix: str

[babacry]

overall looks good but lets make the tag a class instead of a string, for example

@attrs.frozen
class SymbolizeTag:
    prefix: str

Good point, done.

[NoureldinYosri]

LGTM%nit

[NoureldinYosri]

lets move this to the constructor of the tag, see the other comment

[NoureldinYosri]

Suggested change

	prefix: str
	prefix: str = attrs.field(validator=lambda s: len(s) > 0)