[tune] Show the name of training func, instead of just ImplicitFunction.

[xwjiang2010]

Why are these changes needed?

See sample output:

(train_mnist pid=39303) WARNING:tensorflow:From /Users/xwjiang/ray/python/ray/tune/examples/tf_distributed_keras_example.py:49: _CollectiveAllReduceStrategyExperimental.__init__ (from tensorflow.python.distribute.collective_all_reduce_strategy) is deprecated and will be removed in a future version.
(train_mnist pid=39303) Instructions for updating:
(train_mnist pid=39303) use distribute.MultiWorkerMirroredStrategy instead
(train_mnist pid=39303) 2021-12-10 17:45:26.471905: I tensorflow/core/platform/cpu_feature_guard.cc:142] This TensorFlow binary is optimized with oneAPI Deep Neural Network Library (oneDNN) to use the following CPU instructions in performance-critical operations:  AVX2 FMA
(train_mnist pid=39303) To enable them in other operations, rebuild TensorFlow with the appropriate compiler flags.
(train_mnist pid=39303) 2021-12-10 17:45:26.476495: I tensorflow/core/distributed_runtime/rpc/grpc_channel.cc:272] Initialize GrpcChannelCache for job worker -> {0 -> 127.0.0.1:62516, 1 -> 127.0.0.1:62518}
(train_mnist pid=39303) 2021-12-10 17:45:26.477083: I tensorflow/core/distributed_runtime/rpc/grpc_server_lib.cc:427] Started server with target: grpc://127.0.0.1:62518
(train_mnist pid=39304) WARNING:tensorflow:From /Users/xwjiang/ray/python/ray/tune/examples/tf_distributed_keras_example.py:49: _CollectiveAllReduceStrategyExperimental.__init__ (from tensorflow.python.distribute.collective_all_reduce_strategy) is deprecated and will be removed in a future version.
(train_mnist pid=39304) Instructions for updating:
(train_mnist pid=39304) use distribute.MultiWorkerMirroredStrategy instead
(train_mnist pid=39304) 2021-12-10 17:45:26.471349: I tensorflow/core/platform/cpu_feature_guard.cc:142] This TensorFlow binary is optimized with oneAPI Deep Neural Network Library (oneDNN) to use the following CPU instructions in performance-critical operations:  AVX2 FMA
(train_mnist pid=39304) To enable them in other operations, rebuild TensorFlow with the appropriate compiler flags.
(train_mnist pid=39304) 2021-12-10 17:45:26.476290: I tensorflow/core/distributed_runtime/rpc/grpc_channel.cc:272] Initialize GrpcChannelCache for job worker -> {0 -> 127.0.0.1:62516, 1 -> 127.0.0.1:62518}
(train_mnist pid=39304) 2021-12-10 17:45:26.476866: I tensorflow/core/distributed_runtime/rpc/grpc_server_lib.cc:427] Started server with target: grpc://127.0.0.1:62516
(train_mnist pid=39303) 2021-12-10 17:45:26.988813: W tensorflow/core/grappler/optimizers/data/auto_shard.cc:695] AUTO sharding policy will apply DATA sharding policy as it failed to apply FILE sharding policy because of the following reason: Found an unshardable source dataset: name: "TensorSliceDataset/_2"
(train_mnist pid=39303) op: "TensorSliceDataset"

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Checks

• I've run scripts/format.sh to lint the changes in this PR.
• I've included any doc changes needed for https://docs.ray.io/en/master/.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
  • Unit tests
  • Release tests
  • This PR is not tested :(