[TENSOR] eager mode support

[qinxuye]

Is your feature request related to a problem? Please describe.

Currently, the tensor can only be executed by the graph mode, we build a tensor graph, submit it to the cluster when users call the execute method.

Although the graph mode is born for performance, it's quite unfriendly for developing, and hard to debug. Thus, I suggest that we can provide an eager mode.

Describe the solution you'd like

We can enable the eager mode by the option.

Enable the eager mode globally.

from mars.config import options

options.eager_mode = True

Or use a context.

from mars.config import option_context

with option_context() as options:
    options.eager_mode = True
    # the eager mode is on only for the with statement

Since every Tensor object is created by the new_tensors method defined in tensor/expressions/core.py, if the eager mode is on, we can trigger the submission graph there to make sure the tensor is executed eagerly.

Additional context

Maybe we can abstract a new_entities not only for the tensors, prepared for the coming dataframe module.

[qinxuye]

I think in order to reach the availability of eager mode, we can separate the develop process into a few steps.

• Record which tensor has been executed in a specified session. (Support fetch executed tensor data from server #80)
• Update the shape and so forth after execute.(Update tensor shape after execution #120)
• Handle the executed tensor by replacing it with tensor whose op is Fetch. Handling logic for local, local cluster and distributed session may be different.(Replace executed tensor with fetch tensor #180)
• Provide a base class for the TensorOperandMixin which has a new_tensors method, maybe we need to implement a new_entities for the base class, and call it inside the new_tensors. This is a preparation for the coming dataframe module. (DataFrame initial support #173)
• If eager mode is on, make the new_tensor(maybe new_entity) trigger the execution when building expressions, we need to take care of the case cautiously, because new_tensor is also widely used in the tile, we need to make sure that the new_tensor in tile would not trigger the execution.(Support eager mode #200)

cc @NJUPole if there is any additional suggestion.

[qinxuye]

As all steps have been achieved, this summary issue can be closed.