//  [python]
def train_step(self, text_embeddings, pred_rgb, guidance_scale=100, as_latent=False, grad_scale=1,
                   save_guidance_path:Path=None):

        if as_latent:
            latents = t(pred_rgb, (64, 64), mode='bilinear', align_corners=False) * 2 - 1
        else:
            # interp to 512x512 to be fed into vae.
            pred_rgb_512 = t(pred_rgb, (512, 512), mode='bilinear', align_corners=False)
            # encode image into latents with vae, requires grad!
            latents = self.encode_imgs(pred_rgb_512)

        # timestep ~ U(0.02, 0.98) to avoid very high/low noise level
        t = torch.randint(self.min_step, self.max_step + 1, (latents.shape[0],), dtype=torch.long, device=self.device)

        # predict the noise residual with unet, NO grad!
        with torch.no_grad():
            # add noise
            noise = torch.randn_like(latents)
            latents_noisy = self.scheduler.add_noise(latents, noise, t)
            # pred noise
            latent_model_input = torch.cat([latents_noisy] * 2)
            tt = torch.cat([t] * 2)
            noise_pred = self.unet(latent_model_input, tt, encoder_hidden_states=text_embeddings).sample

            # perform guidance (high scale from paper!)
            noise_pred_uncond, noise_pred_pos = noise_pred.chunk(2)
            noise_pred = noise_pred_uncond + guidance_scale * (noise_pred_pos - noise_pred_uncond)

        # import kiui
        # latents_tmp = torch.randn((1, 4, 64, 64), device=self.device)
        # latents_tmp = latents_tmp.detach()
        # kiui.lo(latents_tmp)
        # self.scheduler.set_timesteps(30)
        # for i, t in enumerate(self.scheduler.timesteps):
        #     latent_model_input = torch.cat([latents_tmp] * 3)
        #     noise_pred = self.unet(latent_model_input, t, encoder_hidden_states=text_embeddings)['sample']
        #     noise_pred_uncond, noise_pred_pos = noise_pred.chunk(2)
        #     noise_pred = noise_pred_uncond + 10 * (noise_pred_pos - noise_pred_uncond)
        #     latents_tmp = self.scheduler.step(noise_pred, t, latents_tmp)['prev_sample']
        # imgs = self.decode_latents(latents_tmp)
        # kiui.vis.plot_image(imgs)

        # w(t), sigma_t^2
        w = (1 - self.alphas[t])
        grad = grad_scale * w[:, None, None, None] * (noise_pred - noise)
        grad = torch.nan_to_num(grad)

        if save_guidance_path:
            with torch.no_grad():
                if as_latent:
                    pred_rgb_512 = self.decode_latents(latents)

                # visualize predicted denoised image
                # The following block of code is equivalent to `predict_start_from_noise`...
                # see zero123_utils.py's version for a simpler implementation.
                alphas = self.scheduler.alphas.to(latents)
                total_timesteps = self.max_step - self.min_step + 1
                index = total_timesteps - t.to(latents.device) - 1 
                b = len(noise_pred)
                a_t = alphas[index].reshape(b,1,1,1).to(self.device)
                sqrt_one_minus_alphas = torch.sqrt(1 - alphas)
                sqrt_one_minus_at = sqrt_one_minus_alphas[index].reshape((b,1,1,1)).to(self.device)                
                pred_x0 = (latents_noisy - sqrt_one_minus_at * noise_pred) / a_t.sqrt() # current prediction for x_0
                result_hopefully_less_noisy_image = self.decode_latents(pred_x0.to(latents.type(self.precision_t)))

                # visualize noisier image
                result_noisier_image = self.decode_latents(latents_noisy.to(pred_x0).type(self.precision_t))

                # TODO: also denoise all-the-way

                # all 3 input images are [1, 3, H, W], e.g. [1, 3, 512, 512]
                viz_images = torch.cat([pred_rgb_512, result_noisier_image, result_hopefully_less_noisy_image],dim=0)
                save_image(viz_images, save_guidance_path)

        targets = (latents - grad).detach()
        loss = 0.5 * F.mse_loss(latents.float(), targets, reduction='sum') / latents.shape[0]

        return loss