Tidy up LSI MNIST notebook (#444)

## Description

<!-- Provide a brief description of the PR's purpose here. -->

Small fixes such as unused imports and variables.

## Status

- [x] I have read the guidelines in

[CONTRIBUTING.md](https://github.com/icaros-usc/pyribs/blob/master/CONTRIBUTING.md)
- [x] I have formatted my code using `yapf`
- [x] I have tested my code by running `pytest`
- [x] I have linted my code with `pylint`
- [x] I have added a one-line description of my change to the changelog
in
      `HISTORY.md`
- [x] This PR is ready to go

HISTORY.md

@@ -50,6 +50,10 @@
 - Add cron job for testing installation ({pr}`389`, {pr}`401`)
 - Fix broken cross-refs in docs ({pr}`393`)
 
+#### Documentation
+
+- Tidy up LSI MNIST notebook ({pr}`444`)
+
 ## 0.6.4
 
 Small release that adds the scalable CMA-MAE tutorial.

tutorials/lsi_mnist.ipynb

@@ -43,14 +43,12 @@
    "metadata": {},
    "outputs": [],
    "source": [
-    "import time\n",
     "import sys\n",
     "\n",
     "import matplotlib.pyplot as plt\n",
     "import numpy as np\n",
     "import torch\n",
     "import torch.nn as nn\n",
-    "import torchvision\n",
     "from tqdm import tqdm, trange"
    ]
   },
@@ -341,8 +339,6 @@
    ],
    "source": [
     "total_itrs = 30_000\n",
-    "flat_img_size = 784  # 28 * 28\n",
-    "start_time = time.time()\n",
     "\n",
     "for itr in trange(1, total_itrs + 1, file=sys.stdout, desc='Iterations'):\n",
     "    sols = scheduler.ask()\n",
@@ -368,7 +364,7 @@
     "\n",
     "        # Shape: len(sols) x 784\n",
     "        flattened_imgs = normalized_imgs.cpu().numpy().reshape(\n",
-    "            (-1, flat_img_size))\n",
+    "            (normalized_imgs.shape[0], -1))\n",
     "\n",
     "        # The first measures is the \"boldness\" of the digit (i.e. number of white\n",
     "        # pixels). We consider pixels with values larger than or equal to 0.5\n",