Add a simple sdpa (#3037)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/3037

Add a simple sdpa so it's decomposed to simpler ops instead of the decompose F.scaled_dot_product_attention, which includes 29 ops including `torch.where`
```
def forward(self, q, k, v):
    aten_mul_scalar = executorch_exir_dialects_edge__ops_aten_mul_Scalar(q, 0.5946035575013605);  q = None
    aten_full_default = executorch_exir_dialects_edge__ops_aten_full_default([8, 8], True, dtype = torch.bool, layout = torch.strided, device = device(type='cpu'), pin_memory = False)
    aten_arange_start_step = executorch_exir_dialects_edge__ops_aten_arange_start_step(0, 8, layout = torch.strided, device = device(type='cpu'), pin_memory = False)
    aten_unsqueeze_copy_default = executorch_exir_dialects_edge__ops_aten_unsqueeze_copy_default(aten_arange_start_step, -2);  aten_arange_start_step = None
    aten_arange_start_step_1 = executorch_exir_dialects_edge__ops_aten_arange_start_step(0, 8, layout = torch.strided, device = device(type='cpu'), pin_memory = False)
    aten_unsqueeze_copy_default_1 = executorch_exir_dialects_edge__ops_aten_unsqueeze_copy_default(aten_arange_start_step_1, -1);  aten_arange_start_step_1 = None
    aten_sub_tensor = executorch_exir_dialects_edge__ops_aten_sub_Tensor(aten_unsqueeze_copy_default, aten_unsqueeze_copy_default_1);  aten_unsqueeze_copy_default = aten_unsqueeze_copy_default_1 = None
    aten_le_scalar = executorch_exir_dialects_edge__ops_aten_le_Scalar(aten_sub_tensor, 0);  aten_sub_tensor = None
    aten_logical_and_default = executorch_exir_dialects_edge__ops_aten_logical_and_default(aten_le_scalar, aten_full_default);  aten_le_scalar = aten_full_default = None
    aten_full_like_default = executorch_exir_dialects_edge__ops_aten_full_like_default(aten_logical_and_default, 0, dtype = torch.float32, pin_memory = False, memory_format = torch.preserve_format)
    aten_logical_not_default = executorch_exir_dialects_edge__ops_aten_logical_not_default(aten_logical_and_default);  aten_logical_and_default = None
    aten_scalar_tensor_default = executorch_exir_dialects_edge__ops_aten_scalar_tensor_default(-inf, dtype = torch.float32, layout = torch.strided, device = device(type='cpu'))
    aten_where_self = executorch_exir_dialects_edge__ops_aten_where_self(aten_logical_not_default, aten_scalar_tensor_default, aten_full_like_default);  aten_logical_not_default = aten_scalar_tensor_default = aten_full_like_default = None
    aten_permute_copy_default = executorch_exir_dialects_edge__ops_aten_permute_copy_default(k, [0, 1, 3, 2]);  k = None
    aten_mul_scalar_1 = executorch_exir_dialects_edge__ops_aten_mul_Scalar(aten_permute_copy_default, 0.5946035575013605);  aten_permute_copy_default = None
    aten_expand_copy_default = executorch_exir_dialects_edge__ops_aten_expand_copy_default(aten_mul_scalar, [1, 1, 8, 8]);  aten_mul_scalar = None
    aten_view_copy_default = executorch_exir_dialects_edge__ops_aten_view_copy_default(aten_expand_copy_default, [1, 8, 8]);  aten_expand_copy_default = None
    aten_expand_copy_default_1 = executorch_exir_dialects_edge__ops_aten_expand_copy_default(aten_mul_scalar_1, [1, 1, 8, 8]);  aten_mul_scalar_1 = None
    aten_view_copy_default_1 = executorch_exir_dialects_edge__ops_aten_view_copy_default(aten_expand_copy_default_1, [1, 8, 8]);  aten_expand_copy_default_1 = None
    aten_bmm_default = executorch_exir_dialects_edge__ops_aten_bmm_default(aten_view_copy_default, aten_view_copy_default_1);  aten_view_copy_default = aten_view_copy_default_1 = None
    aten_view_copy_default_2 = executorch_exir_dialects_edge__ops_aten_view_copy_default(aten_bmm_default, [1, 1, 8, 8]);  aten_bmm_default = None
    aten_add_tensor = executorch_exir_dialects_edge__ops_aten_add_Tensor(aten_view_copy_default_2, aten_where_self);  aten_view_copy_default_2 = aten_where_self = None
    aten__softmax_default = executorch_exir_dialects_edge__ops_aten__softmax_default(aten_add_tensor, -1, False);  aten_add_tensor = None
    aten_expand_copy_default_2 = executorch_exir_dialects_edge__ops_aten_expand_copy_default(aten__softmax_default, [1, 1, 8, 8]);  aten__softmax_default = None
    aten_view_copy_default_3 = executorch_exir_dialects_edge__ops_aten_view_copy_default(aten_expand_copy_default_2, [1, 8, 8]);  aten_expand_copy_default_2 = None
    aten_expand_copy_default_3 = executorch_exir_dialects_edge__ops_aten_expand_copy_default(v, [1, 1, 8, 8]);  v = None
    aten_view_copy_default_4 = executorch_exir_dialects_edge__ops_aten_view_copy_default(aten_expand_copy_default_3, [1, 8, 8]);  aten_expand_copy_default_3 = None
    aten_bmm_default_1 = executorch_exir_dialects_edge__ops_aten_bmm_default(aten_view_copy_default_3, aten_view_copy_default_4);  aten_view_copy_default_3 = aten_view_copy_default_4 = None
    aten_view_copy_default_5 = executorch_exir_dialects_edge__ops_aten_view_copy_default(aten_bmm_default_1, [1, 1, 8, 8]);  aten_bmm_default_1 = None
    return (aten_view_copy_default_5,)
```
After applying the diff, we remove the following ops
```
    %aten_full_like_default : [num_users=1] = call_function[target=executorch.exir.dialects.edge._ops.aten.full_like.default](args = (%aten_index_tensor_2, 0), kwargs = {dtype: torch.float32, pin_memory: False, memory_format: torch.preserve_format})

    %aten_logical_not_default : [num_users=1] = call_function[target=executorch.exir.dialects.edge._ops.aten.logical_not.default](args = (%aten_index_tensor_2,), kwargs = {})

    %aten_scalar_tensor_default : [num_users=1] = call_function[target=executorch.exir.dialects.edge._ops.aten.scalar_tensor.default](args = (-inf,), kwargs = {dtype: torch.float32, layout: torch.strided, device: cpu})

    %aten_where_self : [num_users=1] = call_function[target=executorch.exir.dialects.edge._ops.aten.where.self](args = (%aten_logical_not_default, %aten_scalar_tensor_default, %aten_full_like_default), kwargs = {})

    %aten_mul_scalar : [num_users=1] = call_function[target=executorch.exir.dialects.edge._ops.aten.mul.Scalar](args = (%aten_permute_copy_default_3, 0.5946035575013605), kwargs = {})
    ...
    %aten_mul_scalar_1 : [num_users=1] = call_function[target=executorch.exir.dialects.edge._ops.aten.mul.Scalar](args = (%aten_permute_copy_default_6, 0.5946035575013605), kwargs = {})
```
but introduce an add
    %aten_add_tensor_3 : [num_users=1] = call_function[target=executorch.exir.dialects.edge._ops.aten.add.Tensor](args = (%aten_mul_tensor_11, %aten_index_tensor_2), kwargs = {})
```
ghstack-source-id: 223152096
exported-using-ghexport

Reviewed By: mergennachin, kimishpatel

Differential Revision: D56119737

fbshipit-source-id: ec8e875f0a4c4ec67b7493e4872c9a5b081e6de7