[nnx] add more decode flags for MultiHeadAttention

[cgarciae]

What does this PR do?

Adds more decode flags for MultiHeadAttention:

• Current decode field is made Optional with a default of None.
• Adds a decode argument to __call__ which can be used to override all other flags.
• __call__ now check if the decode flag is set in nnx.flags as a fallback in case neither the field or the __call__ arguments are passed.
• If no flag is set, the final decode value defaults to False.

[cgarciae]

Continued in #3607