PredicateVector::is_zero ignores the last storage word when kBytes is a multiple of sizeof(Storage)

Description

PredicateVector::is_zero() (include/cutlass/predicate_vector.h ~line 515) masks the last storage word with computeLastWordMask(), which returns 0 whenever the byte count is an exact multiple of sizeof(Storage):

constexpr Storage computeLastWordMask() {
    Storage mask(0);
    for (int byte = 0; byte < kBytes % sizeof(Storage); ++byte)
        mask |= (kByteMask << (byte * 8));
    return mask;
}

For PredicateVector<32> (kPredicatesPerByte=4): kBytes == 8, sizeof(Storage) == 4, so kBytes % sizeof(Storage) == 0 and last_word_mask == 0 - the entire second storage word is excluded from is_zero(). Bits 16..31 can be set and is_zero() still reports true:

kBytes=8 kWordCount=2 sizeof(Storage)=4
is_zero with bits 16..31 set: 1   (expected 0)

The same configuration is reachable for any kPredicates where ceil(kPredicates/4) is a nonzero multiple of 4 (e.g. 16, 32, 48, 64 predicates with default parameters).

Related latent defect in the same header: Iterator::operator- (~line 278) constructs a ConstIterator ret(*this); ... return ret; but declares the return type Iterator, which does not compile if instantiated; and computeLastWordMask()'s loop bound also means configurations where the last word holds fewer than four predicate bytes are handled correctly only by that mask path.

Suggested fix

When kBytes % sizeof(Storage) == 0, the last word should use the full-word mask (computeWordMask()), i.e. select the mask based on whether the last word is partially occupied, for example:

constexpr Storage last_word_mask =
    (kBytes % sizeof(Storage) != 0) ? computeLastWordMask() : computeWordMask();
result |= (storage(kWordCount - 1) & last_word_mask);

Start in include/cutlass/predicate_vector.h around PredicateVector::is_zero() and computeLastWordMask(), then reproduce the PredicateVector<32> case described in the issue with bits 16..31 set. Done means is_zero() reports false for nonzero words, including exact storage-word multiples; also check the mentioned Iterator::operator- return type if addressing that related defect.