Fix deprecation warnings for method-style differential operators.

[dan-zheng]

Before:

/swift-models/Gym/CartPole/main.swift:182:13: warning: 'gradient(in:)' is deprecated: Method-style differential operators are deprecated and will be removed; use top-level
function 'TensorFlow.gradient(at:in:)' instead
        net.gradient { net -> Tensor<Float> in
            ^

After:

/swift-models/Gym/CartPole/main.swift:182:13: warning: 'gradient(in:)' is deprecated: Method-style differential operators are deprecated and will be removed; use top-level function 'TensorFlow.gradient(at:in:)' instead
        net.gradient { net -> Tensor<Float> in
            ^