Forecast retrieval without calling simulate

[Tezirg]

Environment

• Grid2op version: 1.1.1
• System: ubuntu20

Bug description

Getting the forecasted prod and load values without calling obs.simulate before raises a KeyError

How to reproduce

Code snippet

import grid2op

print (grid2op.__version__)
env = grid2op.make("l2rpn_wcci_2020")

# Get forecast after a simulate works                                           
obs = env.reset()
obs.simulate(env.action_space({}))
prod_p_f, prod_v_f, load_p_f, load_q_f = obs.get_forecasted_inj()
print (prod_p_f.shape, prod_v_f.shape, load_p_f.shape, load_q_f.shape)

# Get forecast before any simulate doesnt work                                  
obs = env.reset()
prod_p_f, prod_v_f, load_p_f, load_q_f = obs.get_forecasted_inj()
print (prod_p_f.shape, prod_v_f.shape, load_p_f.shape, load_q_f.shape)

Current output

1.1.1
(22,) (22,) (37,) (37,)
Traceback (most recent call last):
  File "./forecast.py", line 13, in <module>
    prod_p_f, prod_v_f, load_p_f, load_q_f = obs.get_forecasted_inj()
  File "XXX/grid2op/Observation/BaseObservation.py", line 666, in get_forecasted_inj
    a = self._forecasted_grid_act[time_step]["inj_action"]
KeyError: 1

Expected output

1.1.1
(22,) (22,) (37,) (37,)
(22,) (22,) (37,) (37,)