Avoid ToString() in Eager's TFE_Execute.

Also use InlinedVector instead of std::vector for non-async path

Before:
Benchmark              Time(ns)        CPU(ns)     Iterations
-------------------------------------------------------------
BM_Execute/0               1895           1898         360200  Execute
BM_Execute/1               1193           1942         358322  ExecuteAsync
BM_ExecuteFunction/0       5812           5825         100000  ExecuteFunction
BM_ExecuteFunction/1       5015           5374         100000  ExecuteFunctionAsync

After:
Benchmark              Time(ns)        CPU(ns)     Iterations
-------------------------------------------------------------
BM_Execute/0               1604           1607         428262  Execute
BM_Execute/1               1150           1765         404821  ExecuteAsync
BM_ExecuteFunction/0       5615           5626         100000  ExecuteFunction
BM_ExecuteFunction/1       5111           5476         100000  ExecuteFunctionAsync
PiperOrigin-RevId: 193218331