Adding gene symbols for GDSC

[Kexin Huang]

Describe the problem
Adding a function for gene symbols in GDSC gene expressions.

Describe the solution you'd like

from tdc.multi_pred import DrugRes
data = DrugRes(name = 'GDSC1')
data.get_gene_symbols()
## ['TSPAN6', 'TNMD', ....]

Similarly for GDSC2.

Additional context
NA

[Kexin Huang]

You can now get the symbols via:

from tdc.multi_pred import DrugRes
data = DrugRes(name = 'GDSC1')
data.get_gene_symbols()