Flatten out_proj in MultiHeadAttention

[z-a-f]

The MHA has an explicit nn.Linear layer for output projections, which is not consistent with the rest of the implementation (s.a. input projections). In addition to that this makes the nn.MultiHeadAttention dependent on the linear implementation, as well as making it a nested module.

Changes:

1. Remove MultiHeadAttention.out_proj
2. Add MultiHeadAttention.out_proj_weight, MultiHeadAttention.out_proj_bias. Add the functional linear for forward
3. Add initialization
4. Change expected string to hide the out_proj
5. Adds forward compatibility to be able to load old models

Potential issues:

• Initialization: nn.Linear initilizes its weight as uniform Kaiming, while this PR uses uniform Xavier. In addition to that, bias in the nn.Linear is uniform based on fan-in/fan-out, while here it is constant 0. This means that numerically this will be different from the original implementation.
  • Option 1: Accept current change -- this is more consistent with the rest of the implementation
  • Option 2: Duplicate initialization logic from Linear -- this is consistent with the initialization from before this PR

Tests

There are no new tests, as no new logic or change in functionality is introduced.

Potentially fixes #60165

cc @mrshenli @pritamdamania87 @zhaojuanmao @satgera @gqchen @aazzolini @osalpekar @jiayisuse @H-Huang @kwen2501 @awgu @penguinwu @fegin @XilunWu @wanchaol @fduwjj @wz337 @tianyu-l @wconstab @yf225 @chauhang @d4l3k

[pytorch-bot]

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[z-a-f]

cc @jerryzh168 @HDCharles I don't think there is a need for the NonDynamicallyQuantizableLinear anymore. However, it is worth checking for quantization logic changes.

[jbschlosser]

This is BC-breaking, no?

[z-a-f]

@jbschlosser It might be for chckpoints: in case a checkpoint is created post-PR, and being loaded into pre-PR. I can add BC-compatibility by saving the weights as out_proj.weight and out_proj.bias with a deprecation warning, but that would mean the checkpoint state-dict will have structure inconsistent with the MHA, wdyt?