def forward(self,
                query,
                key,
                value,
                residual=None,
                query_pos=None,
                key_padding_mask=None,
                reference_points=None,
                spatial_shapes=None,
                reference_points_cam=None,
                bev_mask=None,
                level_start_index=None,
                flag='encoder',
                **kwargs):
        """Forward Function of Detr3DCrossAtten.
        Args:
            query (Tensor): Query of Transformer with shape
                (num_query, bs, embed_dims).
            key (Tensor): The key tensor with shape
                `(num_key, bs, embed_dims)`.
            value (Tensor): The value tensor with shape
                `(num_key, bs, embed_dims)`. (B, N, C, H, W)
            residual (Tensor): The tensor used for addition, with the
                same shape as `x`. Default None. If None, `x` will be used.
            query_pos (Tensor): The positional encoding for `query`.
                Default: None.
            key_pos (Tensor): The positional encoding for  `key`. Default
                None.
            reference_points (Tensor):  The normalized reference
                points with shape (bs, num_query, 4),
                all elements is range in [0, 1], top-left (0,0),
                bottom-right (1, 1), including padding area.
                or (N, Length_{query}, num_levels, 4), add
                additional two dimensions is (w, h) to
                form reference boxes.
            key_padding_mask (Tensor): ByteTensor for `query`, with
                shape [bs, num_key].
            spatial_shapes (Tensor): Spatial shape of features in
                different level. With shape  (num_levels, 2),
                last dimension represent (h, w).
            level_start_index (Tensor): The start index of each level.
                A tensor has shape (num_levels) and can be represented
                as [0, h_0*w_0, h_0*w_0+h_1*w_1, ...].
        Returns:
             Tensor: forwarded results with shape [num_query, bs, embed_dims].
        """

        if key is None:
            key = query
        if value is None:
            value = key

        if residual is None:
            inp_residual = query
            slots = torch.zeros_like(query)
        if query_pos is not None:
            query = query + query_pos

        bs, num_query, _ = query.size()

        D = reference_points_cam.size(3)
        indexes = []
        for i, mask_per_img in enumerate(bev_mask):
            index_query_per_img = mask_per_img[0].sum(-1).nonzero().squeeze(-1)
            indexes.append(index_query_per_img)
        max_len = max([len(each) for each in indexes])

        # each camera only interacts with its corresponding BEV queries. This step can  greatly save GPU memory.
        queries_rebatch = query.new_zeros(
            [bs, self.num_cams, max_len, self.embed_dims])
        reference_points_rebatch = reference_points_cam.new_zeros(
            [bs, self.num_cams, max_len, D, 2])
        
        for j in range(bs):
            for i, reference_points_per_img in enumerate(reference_points_cam):   
                index_query_per_img = indexes[i]
                queries_rebatch[j, i, :len(index_query_per_img)] = query[j, index_query_per_img]
                reference_points_rebatch[j, i, :len(index_query_per_img)] = reference_points_per_img[j, index_query_per_img]

        num_cams, l, bs, embed_dims = key.shape

        key = key.permute(2, 0, 1, 3).reshape(
            bs * self.num_cams, l, self.embed_dims)
        value = value.permute(2, 0, 1, 3).reshape(
            bs * self.num_cams, l, self.embed_dims)

        queries = self.deformable_attention(query=queries_rebatch.view(bs*self.num_cams, max_len, self.embed_dims), key=key, value=value,
                                            reference_points=reference_points_rebatch.view(bs*self.num_cams, max_len, D, 2), spatial_shapes=spatial_shapes,
                                            level_start_index=level_start_index).view(bs, self.num_cams, max_len, self.embed_dims)
        for j in range(bs):
            for i, index_query_per_img in enumerate(indexes):
                slots[j, index_query_per_img] += queries[j, i, :len(index_query_per_img)]

        count = bev_mask.sum(-1) > 0
        count = count.permute(1, 2, 0).sum(-1)
        count = torch.clamp(count, min=1.0)
        slots = slots / count[..., None]
        slots = self.output_proj(slots)

        return self.dropout(slots) + inp_residual
Forward Function of Detr3DCrossAtten.
        Args:
            query (Tensor): Query of Transformer with shape
                (num_query, bs, embed_dims).
            key (Tensor): The key tensor with shape
                `(num_key, bs, embed_dims)`.
            value (Tensor): The value tensor with shape
                `(num_key, bs, embed_dims)`. (B, N, C, H, W)
            residual (Tensor): The tensor used for addition, with the
                same shape as `x`. Default None. If None, `x` will be used.
            query_pos (Tensor): The positional encoding for `query`.
                Default: None.
            key_pos (Tensor): The positional encoding for  `key`. Default
                None.
            reference_points (Tensor):  The normalized reference
                points with shape (bs, num_query, 4),
                all elements is range in [0, 1], top-left (0,0),
                bottom-right (1, 1), including padding area.
                or (N, Length_{query}, num_levels, 4), add
                additional two dimensions is (w, h) to
                form reference boxes.
            key_padding_mask (Tensor): ByteTensor for `query`, with
                shape [bs, num_key].
            spatial_shapes (Tensor): Spatial shape of features in
                different level. With shape  (num_levels, 2),
                last dimension represent (h, w).
            level_start_index (Tensor): The start index of each level.
                A tensor has shape (num_levels) and can be represented
                as [0, h_0*w_0, h_0*w_0+h_1*w_1, ...].
        Returns:
             Tensor: forwarded results with shape [num_query, bs, embed_dims].
        
projects/mmdet3d_plugin/bevformer/modules/spatial_cross_attention.py