CPU version should be default compile with MKL

[usamec]

Tensorflow compiled with MKL is usually much faster. We should have option for this. https://www.tensorflow.org/guide/performance/overview

[adamcrume]

We can't compile with MKL by default, because that's processor-specific. You can compile with MKL by either setting the TF_RUST_BAZEL_OPTS environment variable to --config=mkl before building, or by compiling TensorFlow manually and installing it where pkg-config can find it.

[usamec]

Great, can we have that env var setting in the documentation?

Although package feature flag would be even better, env var is enough. Thanks.