Before, we had `export.poly_spec` to create a jax.ShapedDtypeStruct`
given a polymorphic shape specification. This function was
invoked `poly_spec(arg_shape, arg_dtype, polymorphic_shape)`.
The `arg_shape` was only needed when the polymorphic shape spec
contained placeholders.
We break out an `export.symbolic_shape` that is just a parser
of polymorphic shape specs and we ask the user to invoke
`jax.ShapeDtypeStruct` directly:
`jax.ShapeDtypeStruct(export.symbolic_shape(polymorphic_shape, like=arg_shape), arg_dtype)`.
We also rename the `export.poly_specs` to `export.arg_specs`.
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