Fix visualize activations (#1211)

Fix visualize activations (Squashed 4 commits by @keineahnung2345)

# Get activations of a few sample layers
activations = model.run_graph([image], [
    ("input_image",        model.keras_model.get_layer("input_image").output)
])
leads to the error:
InvalidArgumentError: input_image:0 is both fed and fetched.

Revise the code according to https://stackoverflow.com/questions/39307108/placeholder-20-is-both-fed-and-fetched

samples/balloon/inspect_balloon_model.ipynb

@@ -1134,7 +1134,7 @@
    "source": [
     "# Get activations of a few sample layers\n",
     "activations = model.run_graph([image], [\n",
-    "    (\"input_image\",        model.keras_model.get_layer(\"input_image\").output),\n",
+    "    (\"input_image\",        tf.identity(model.keras_model.get_layer(\"input_image\").output)),\n",
     "    (\"res2c_out\",          model.keras_model.get_layer(\"res2c_out\").output),\n",
     "    (\"res3c_out\",          model.keras_model.get_layer(\"res3c_out\").output),\n",
     "    (\"res4w_out\",          model.keras_model.get_layer(\"res4w_out\").output),  # for resnet100\n",

samples/coco/inspect_model.ipynb

@@ -1249,7 +1249,7 @@
    "source": [
     "# Get activations of a few sample layers\n",
     "activations = model.run_graph([image], [\n",
-    "    (\"input_image\",        model.keras_model.get_layer(\"input_image\").output),\n",
+    "    (\"input_image\",        tf.identity(model.keras_model.get_layer(\"input_image\").output)),\n",
     "    (\"res4w_out\",          model.keras_model.get_layer(\"res4w_out\").output),  # for resnet100\n",
     "    (\"rpn_bbox\",           model.keras_model.get_layer(\"rpn_bbox\").output),\n",
     "    (\"roi\",                model.keras_model.get_layer(\"ROI\").output),\n",

samples/nucleus/inspect_nucleus_model.ipynb

@@ -1229,7 +1229,7 @@
    "source": [
     "# Get activations of a few sample layers\n",
     "activations = model.run_graph([image], [\n",
-    "    (\"input_image\",        model.keras_model.get_layer(\"input_image\").output),\n",
+    "    (\"input_image\",        tf.identity(model.keras_model.get_layer(\"input_image\").output)),\n",
     "    (\"res2c_out\",          model.keras_model.get_layer(\"res2c_out\").output),\n",
     "    (\"res3c_out\",          model.keras_model.get_layer(\"res3c_out\").output),\n",
     "    (\"rpn_bbox\",           model.keras_model.get_layer(\"rpn_bbox\").output),\n",