Fix warnings in train_face_mask_detector.py

[senofsky]

python scripts/train_face_mask_detector.py -d dataset will print the following warnings during execution:

/home/senofsky/projects/face-mask-detector/venv/lib/python3.8/site-packages/tensorflow/python/ops/random_ops.py:285: SyntaxWarning: "is" with a literal. Did you mean "=="?  minval_is_zero = minval is 0  # pylint: disable=literal-comparison
/home/senofsky/projects/face-mask-detector/venv/lib/python3.8/site-packages/tensorflow/python/ops/random_ops.py:286: SyntaxWarning: "is" with a literal. Did you mean "=="?  maxval_is_one = maxval is 1  # pylint: disable=literal-comparison
/home/senofsky/projects/face-mask-detector/venv/lib/python3.8/site-packages/tensorflow/python/ops/ragged/ragged_batch_gather_with_default_op.py:84: SyntaxWarning: "is not" with a literal. Did you mean "!="?  if (default_value.shape.ndims is not 0
/home/senofsky/projects/face-mask-detector/venv/lib/python3.8/site-packages/tensorflow/python/ops/ragged/ragged_batch_gather_with_default_op.py:85: SyntaxWarning: "is not" with a literal. Did you mean "!="?  and default_value.shape.ndims is not 1):
WARNING:tensorflow:`input_shape` is undefined or non-square, or `rows` is not in [96, 128, 160, 192, 224]. Weights for input shape (224, 224) will be loaded as the default.WARNING:`input_shape` is undefined or non-square, or `rows` is not in [96, 128, 160, 192, 224]. Weights for input shape (224, 224) will be loaded as the default.
2020-07-16 19:26:24.988178: W tensorflow/stream_executor/platform/default/dso_loader.cc:55] Could not load dynamic library 'libcuda.so.1'; dlerror: libcuda.so.1: cannot open shared object file: No such file or directory
2020-07-16 19:26:24.988200: E tensorflow/stream_executor/cuda/cuda_driver.cc:313] failed call to cuInit: UNKNOWN ERROR (303)
2020-07-16 19:26:24.988219: I tensorflow/stream_executor/cuda/cuda_diagnostics.cc:156] kernel driver does not appear to be running on this host (spirit-of-fire): /proc/driver/nvidia/version does not exist2020-07-16 19:26:24.988390: I tensorflow/core/platform/cpu_feature_guard.cc:143] Your CPU supports instructions that this TensorFlow binary was not compiled to use: AVX2 FMA
2020-07-16 19:26:25.009288: I tensorflow/core/platform/profile_utils/cpu_utils.cc:102] CPU Frequency: 3001000000 Hz
2020-07-16 19:26:25.009793: I tensorflow/compiler/xla/service/service.cc:168] XLA service 0x7f4a2c000b60 initialized for platform Host (this does not guarantee that XLA will be used). Devices:2020-07-16 19:26:25.009812: I tensorflow/compiler/xla/service/service.cc:176]   StreamExecutor device (0): Host, Default Version