from transformers import PretrainedConfig, PreTrainedModel, AutoModelForCausalLM # Import AutoModelForCausalLM
import torch
import torch.nn as nn
import torch.nn.functional as F
import math
from transformers.modeling_outputs import CausalLMOutputWithPast # Import the necessary output class

# Define the Cross-Expert Attention mechanism
class CrossExpertAttention(nn.Module):
    def __init__(self, config: MeshConfig):
        super().__init__()
        self.config = config
        # Define multi-head attention layers or similar for cross-expert communication
        # This is a placeholder and needs detailed implementation
        self.cross_attention = nn.MultiheadAttention(
            embed_dim=config.hidden_size,
            num_heads=config.num_attention_heads, # Using model's attention heads for now
            batch_first=True
        )

    def forward(self, expert_outputs):
        # expert_outputs shape: (batch_size, sequence_length, num_experts, hidden_size)

        if not self.config.cross_expert_attention_enabled:
            return expert_outputs

        # Reshape for attention: (batch_size * sequence_length, num_experts, hidden_size)
        batch_seq_size = expert_outputs.shape[0] * expert_outputs.shape[1]
        reshaped_outputs = expert_outputs.view(batch_seq_size, self.config.mesh_grid_size[0] * self.config.mesh_grid_size[1], self.config.hidden_size)

        # Apply cross-expert attention. Query, Key, Value are the same here (self-attention across experts)
        # Attention mask could be used to restrict communication if needed
        cross_attn_output, _ = self.cross_attention(reshaped_outputs, reshaped_outputs, reshaped_outputs)

        # Reshape back: (batch_size, sequence_length, num_experts, hidden_size)
        cross_attn_output = cross_attn_output.view(
            expert_outputs.shape[0], expert_outputs.shape[1], self.config.mesh_grid_size[0] * self.config.mesh_grid_size[1], self.config.hidden_size
        )

        return cross_attn_output