Add MultiheadAttention to DirectMLX

[PatriceVignola]

No description provided.

[fdwr]

    inline MultiheadAttentionOutputs MultiheadAttention(

Multihead is a single word (https://en.wiktionary.org/wiki/multihead / https://www.merriam-webster.com/dictionary/multiheaded), consistent with our enum DML_MULTIHEAD_ATTENTION_OPERATOR_DESC and with PyTorch (https://pytorch.org/docs/stable/generated/torch.nn.MultiheadAttention.html). The people using hyphens don't know better that "multi" is a prefix :b.

[fdwr]

(minor 🤷) assert(stackedKeyValue || (key && value)); for assertion consistency with the other branches?

[fdwr]

stackedQueryKeyValueTensor.sizes.size() - 5

I worry about callers using DMLX and directly populating tensors from some model description, and then DMLX accessing invalid negative indices here because the tensor size is too small, especially if that model description comes from external data that is not completely under the program's control. We could say that it's the responsibility of the caller to validate all these sizes up-front before calling DMLX, but even DML validates tensor sizes before accessing any potentially invalid indices. Can we strengthen these mere asserts which only happen in debug builds to an std::invalid_argument instead?

e.g.

            DMLX_THROW_IF_NOT(stackedQueryKeyValueTensor.sizes.size() >= 5, std::invalid_argument);
            batchSize = stackedQueryKeyValueTensor.sizes[stackedQueryKeyValueTensor.sizes.size() - 5];

#if __cpp_exceptions
    #if DMLX_USE_WIL
        #define DMLX_THROW_IF_FAILED(_hr) THROW_IF_FAILED(_hr)
        #define DMLX_THROW(_hr) THROW_HR(_hr)
        #define DMLX_THROW_IF_NOT(condition, exceptionType) if (!(condition)) { throw exceptionType; }
    #else
        #define DMLX_THROW_IF_FAILED(_hr) if (FAILED(_hr)) { throw std::runtime_error(#_hr); }
        #define DMLX_THROW(_hr) throw std::runtime_error(#_hr); 
        #define DMLX_THROW_IF_NOT(condition, exceptionType) if (!(condition)) { throw exceptionType; }
    #endif
#else
    #define DMLX_THROW_IF_FAILED(_hr) if (FAILED(_hr)) { std::abort(); }
    #define DMLX_THROW(_hr) { std::abort(); } 
    #define DMLX_THROW_IF_NOT(condition, exceptionType) { std::abort(); } 
#endif

I'm not proposing we turn every assert into an exception, as DML API validation will validate things too, and we don't need to doubly validate in DMLX, but at least to validate the cases where DMLX itself would access invalid memory.

[fdwr]

3 comments. LGTM otherwise.