CuTeDSL: width-only f-string format spec emits a malformed device printf format (%8) and silently drops the value

Description

FormattedValue.to_str in cutlass/base_dsl/ast_helpers.py (~lines 731-737) converts a non-empty single-token format_spec into a printf conversion by emitting "%{format_spec[0]}". That is only valid when the spec actually ends in a conversion character. A width-only spec such as {x:8} is legal Python but produces the literal format %8, which consumes no argument and prints garbage.

Unit-level repro (against nvidia-cutlass-dsl==4.7.0):

from cutlass.base_dsl.ast_helpers import FormattedValue
import cutlass

fv = FormattedValue(value=cutlass.Int32(7), conversion=-1, format_spec=["8"])
print(fv.to_str()[0])    # prints: %8
fv = FormattedValue(value=cutlass.Int32(7), conversion=-1, format_spec=[".2f"])
print(fv.to_str()[0])    # prints: %.2f  (fine)

End-to-end, cute.printf(f"[{x:8}]") with a dynamic x makes the device print [ 8]-style text where the value never appears: the kernel receives format string [%8] plus an unconsumed vararg. No diagnostic is raised.

Suggested fix

Validate that the spec terminates in a known conversion character. Either raise DiagId.UNSUP_FSTRING_FORMAT for specs without one, or map a bare numeric spec to a sensible default conversion per the value's dtype (e.g. append d for integer types, f/g for float types) so {x:8} keeps its Python meaning.